Allow serving API only via new cli command

- Add new command to the cli/node: `serveapi` that alows serving the API just
  by connecting to the PostgreSQL database.  The mode flag should me passed in
  order to select whether we are connecting to a synchronizer database or a
  coordinator database.  If `coord` is chosen as mode, the coordinator
  endpoints can be activated in order to allow inserting l2txs and
  authorizations into the L2DB.

Summary of the implementation details
- New SQL table with 3 columns (plus `item_id` pk).  The table only contains a
  single row with `item_id` = 1.  Columns:
    - state: historydb.StateAPI in JSON.  This is the struct that is served via
      the `/state` API endpoint.  The node will periodically update this struct
      and store it int he DB.  The api server will query it from the DB to
      serve it.
    - config: historydb.NodeConfig in JSON.  This struct contains node
      configuration parameters that the API needs to be aware of.  It's updated
      once every time the node starts.
    - constants: historydb.Constants in JSON.  This struct contains all the
      hermez network constants gathered via the ethereum client by the node.
      It's written once every time the node starts.
- The HistoryDB contains methods to get and update each one of these columns
  individually.
- The HistoryDB contains all methods that query the DB and prepare objects that
  will appear in the StateAPI endpoint.
- The configuration used in for the `serveapi` cli/node command is defined in
  `config.APIServer`, and is a subset of `node.Config` in order to allow
  reusing the same configuration file of the node if desired.
- A new object is introduced in the api: `StateAPIUpdater`, which contains all
  the necessary information to update the StateAPI in the DB periodically by
  the node.

- Moved the types `SCConsts`, `SCVariables` and `SCVariablesPtr` from
  `syncrhonizer` to `common` for convenience.

stateapiupdater/stateapiupdater.go

@@ -0,0 +1,155 @@
+package stateapiupdater
+
+import (
+	"database/sql"
+	"sync"
+
+	"github.com/hermeznetwork/hermez-node/common"
+	"github.com/hermeznetwork/hermez-node/db/historydb"
+	"github.com/hermeznetwork/tracerr"
+)
+
+// Updater is an utility object to facilitate updating the StateAPI
+type Updater struct {
+	hdb    *historydb.HistoryDB
+	state  historydb.StateAPI
+	config historydb.NodeConfig
+	vars   common.SCVariablesPtr
+	consts historydb.Constants
+	rw     sync.RWMutex
+}
+
+// NewUpdater creates a new Updater
+func NewUpdater(hdb *historydb.HistoryDB, config *historydb.NodeConfig, vars *common.SCVariables,
+	consts *historydb.Constants) *Updater {
+	u := Updater{
+		hdb:    hdb,
+		config: *config,
+		consts: *consts,
+		state: historydb.StateAPI{
+			NodePublicConfig: historydb.NodePublicConfig{
+				ForgeDelay: config.ForgeDelay,
+			},
+		},
+	}
+	u.SetSCVars(vars.AsPtr())
+	return &u
+}
+
+// Store the State in the HistoryDB
+func (u *Updater) Store() error {
+	u.rw.RLock()
+	defer u.rw.RUnlock()
+	return tracerr.Wrap(u.hdb.SetStateInternalAPI(&u.state))
+}
+
+// SetSCVars sets the smart contract vars (ony updates those that are not nil)
+func (u *Updater) SetSCVars(vars *common.SCVariablesPtr) {
+	u.rw.Lock()
+	defer u.rw.Unlock()
+	if vars.Rollup != nil {
+		u.vars.Rollup = vars.Rollup
+		rollupVars := historydb.NewRollupVariablesAPI(u.vars.Rollup)
+		u.state.Rollup = *rollupVars
+	}
+	if vars.Auction != nil {
+		u.vars.Auction = vars.Auction
+		auctionVars := historydb.NewAuctionVariablesAPI(u.vars.Auction)
+		u.state.Auction = *auctionVars
+	}
+	if vars.WDelayer != nil {
+		u.vars.WDelayer = vars.WDelayer
+		u.state.WithdrawalDelayer = *u.vars.WDelayer
+	}
+}
+
+// UpdateRecommendedFee update Status.RecommendedFee information
+func (u *Updater) UpdateRecommendedFee() error {
+	recommendedFee, err := u.hdb.GetRecommendedFee(u.config.MinFeeUSD)
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
+	u.rw.Lock()
+	u.state.RecommendedFee = *recommendedFee
+	u.rw.Unlock()
+	return nil
+}
+
+// UpdateMetrics update Status.Metrics information
+func (u *Updater) UpdateMetrics() error {
+	u.rw.RLock()
+	lastBatch := u.state.Network.LastBatch
+	u.rw.RUnlock()
+	if lastBatch == nil {
+		return nil
+	}
+	lastBatchNum := lastBatch.BatchNum
+	metrics, err := u.hdb.GetMetricsInternalAPI(lastBatchNum)
+	if err != nil {
+		return tracerr.Wrap(err)
+	}
+	u.rw.Lock()
+	u.state.Metrics = *metrics
+	u.rw.Unlock()
+	return nil
+}
+
+// UpdateNetworkInfoBlock update Status.Network block related information
+func (u *Updater) UpdateNetworkInfoBlock(lastEthBlock, lastSyncBlock common.Block) {
+	u.rw.Lock()
+	u.state.Network.LastSyncBlock = lastSyncBlock.Num
+	u.state.Network.LastEthBlock = lastEthBlock.Num
+	u.rw.Unlock()
+}
+
+// UpdateNetworkInfo update Status.Network information
+func (u *Updater) UpdateNetworkInfo(
+	lastEthBlock, lastSyncBlock common.Block,
+	lastBatchNum common.BatchNum, currentSlot int64,
+) error {
+	// Get last batch in API format
+	lastBatch, err := u.hdb.GetBatchInternalAPI(lastBatchNum)
+	if tracerr.Unwrap(err) == sql.ErrNoRows {
+		lastBatch = nil
+	} else if err != nil {
+		return tracerr.Wrap(err)
+	}
+	u.rw.RLock()
+	auctionVars := u.vars.Auction
+	u.rw.RUnlock()
+	// Get next forgers
+	lastClosedSlot := currentSlot + int64(auctionVars.ClosedAuctionSlots)
+	nextForgers, err := u.hdb.GetNextForgersInternalAPI(auctionVars, &u.consts.Auction,
+		lastSyncBlock, currentSlot, lastClosedSlot)
+	if tracerr.Unwrap(err) == sql.ErrNoRows {
+		nextForgers = nil
+	} else if err != nil {
+		return tracerr.Wrap(err)
+	}
+
+	bucketUpdates, err := u.hdb.GetBucketUpdatesInternalAPI()
+	if err == sql.ErrNoRows {
+		bucketUpdates = nil
+	} else if err != nil {
+		return tracerr.Wrap(err)
+	}
+
+	u.rw.Lock()
+	// Update NodeInfo struct
+	for i, bucketParams := range u.state.Rollup.Buckets {
+		for _, bucketUpdate := range bucketUpdates {
+			if bucketUpdate.NumBucket == i {
+				bucketParams.Withdrawals = bucketUpdate.Withdrawals
+				u.state.Rollup.Buckets[i] = bucketParams
+				break
+			}
+		}
+	}
+	u.state.Network.LastSyncBlock = lastSyncBlock.Num
+	u.state.Network.LastEthBlock = lastEthBlock.Num
+	u.state.Network.LastBatch = lastBatch
+	u.state.Network.CurrentSlot = currentSlot
+	u.state.Network.NextForgers = nextForgers
+	u.rw.Unlock()
+	return nil
+}