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.

common/eth.go

@@ -0,0 +1,33 @@
+package common
+
+// SCVariables joins all the smart contract variables in a single struct
+type SCVariables struct {
+	Rollup   RollupVariables   `validate:"required"`
+	Auction  AuctionVariables  `validate:"required"`
+	WDelayer WDelayerVariables `validate:"required"`
+}
+
+// AsPtr returns the SCVariables as a SCVariablesPtr using pointers to the
+// original SCVariables
+func (v *SCVariables) AsPtr() *SCVariablesPtr {
+	return &SCVariablesPtr{
+		Rollup:   &v.Rollup,
+		Auction:  &v.Auction,
+		WDelayer: &v.WDelayer,
+	}
+}
+
+// SCVariablesPtr joins all the smart contract variables as pointers in a single
+// struct
+type SCVariablesPtr struct {
+	Rollup   *RollupVariables   `validate:"required"`
+	Auction  *AuctionVariables  `validate:"required"`
+	WDelayer *WDelayerVariables `validate:"required"`
+}
+
+// SCConsts joins all the smart contract constants in a single struct
+type SCConsts struct {
+	Rollup   RollupConstants
+	Auction  AuctionConstants
+	WDelayer WDelayerConstants
+}

common/fee.go

@@ -22,9 +22,9 @@ var FeeFactorLsh60 [256]*big.Int
 // the coordinator according to the tx type (if the tx requires to create an
 // account and register, only register or he account already esists)
 type RecommendedFee struct {
-	ExistingAccount           float64 `json:"existingAccount"`
-	CreatesAccount            float64 `json:"createAccount"`
-	CreatesAccountAndRegister float64 `json:"createAccountInternal"`
+	ExistingAccount        float64 `json:"existingAccount"`
+	CreatesAccount         float64 `json:"createAccount"`
+	CreatesAccountInternal float64 `json:"createAccountInternal"`
 }
 
 // FeeSelector is used to select a percentage from the FeePlan.