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.

cli/node/cfg.api.toml

@@ -0,0 +1,24 @@
+[API]
+Address = "localhost:8386"
+Explorer = true
+MaxSQLConnections = 10
+SQLConnectionTimeout = "2s"
+
+[PostgreSQL]
+PortWrite     = 5432
+HostWrite     = "localhost"
+UserWrite     = "hermez"
+PasswordWrite = "yourpasswordhere"
+NameWrite     = "hermez"
+
+[Coordinator.L2DB]
+SafetyPeriod = 10
+MaxTxs       = 512
+TTL          = "24h"
+PurgeBatchDelay = 10
+InvalidateBatchDelay = 20
+PurgeBlockDelay = 10
+InvalidateBlockDelay = 20
+
+[Coordinator.API]
+Coordinator = true

cli/node/main.go

@@ -196,17 +196,7 @@ func cmdWipeSQL(c *cli.Context) error {
 	return nil
 }
 
-func cmdRun(c *cli.Context) error {
-	cfg, err := parseCli(c)
-	if err != nil {
-		return tracerr.Wrap(fmt.Errorf("error parsing flags and config: %w", err))
-	}
-	node, err := node.NewNode(cfg.mode, cfg.node)
-	if err != nil {
-		return tracerr.Wrap(fmt.Errorf("error starting node: %w", err))
-	}
-	node.Start()
-
+func waitSigInt() {
 	stopCh := make(chan interface{})
 
 	// catch ^C to send the stop signal
@@ -227,11 +217,40 @@ func cmdRun(c *cli.Context) error {
 		}
 	}()
 	<-stopCh
+}
+
+func cmdRun(c *cli.Context) error {
+	cfg, err := parseCli(c)
+	if err != nil {
+		return tracerr.Wrap(fmt.Errorf("error parsing flags and config: %w", err))
+	}
+	node, err := node.NewNode(cfg.mode, cfg.node)
+	if err != nil {
+		return tracerr.Wrap(fmt.Errorf("error starting node: %w", err))
+	}
+	node.Start()
+	waitSigInt()
 	node.Stop()
 
 	return nil
 }
 
+func cmdServeAPI(c *cli.Context) error {
+	cfg, err := parseCliAPIServer(c)
+	if err != nil {
+		return tracerr.Wrap(fmt.Errorf("error parsing flags and config: %w", err))
+	}
+	srv, err := node.NewAPIServer(cfg.mode, cfg.server)
+	if err != nil {
+		return tracerr.Wrap(fmt.Errorf("error starting api server: %w", err))
+	}
+	srv.Start()
+	waitSigInt()
+	srv.Stop()
+
+	return nil
+}
+
 func cmdDiscard(c *cli.Context) error {
 	_cfg, err := parseCli(c)
 	if err != nil {
@@ -319,20 +338,59 @@ func getConfig(c *cli.Context) (*Config, error) {
 	var cfg Config
 	mode := c.String(flagMode)
 	nodeCfgPath := c.String(flagCfg)
-	if nodeCfgPath == "" {
-		return nil, tracerr.Wrap(fmt.Errorf("required flag \"%v\" not set", flagCfg))
-	}
 	var err error
 	switch mode {
 	case modeSync:
 		cfg.mode = node.ModeSynchronizer
-		cfg.node, err = config.LoadNode(nodeCfgPath)
+		cfg.node, err = config.LoadNode(nodeCfgPath, false)
 		if err != nil {
 			return nil, tracerr.Wrap(err)
 		}
 	case modeCoord:
 		cfg.mode = node.ModeCoordinator
-		cfg.node, err = config.LoadCoordinator(nodeCfgPath)
+		cfg.node, err = config.LoadNode(nodeCfgPath, true)
+		if err != nil {
+			return nil, tracerr.Wrap(err)
+		}
+	default:
+		return nil, tracerr.Wrap(fmt.Errorf("invalid mode \"%v\"", mode))
+	}
+
+	return &cfg, nil
+}
+
+// ConfigAPIServer is the configuration of the api server execution
+type ConfigAPIServer struct {
+	mode   node.Mode
+	server *config.APIServer
+}
+
+func parseCliAPIServer(c *cli.Context) (*ConfigAPIServer, error) {
+	cfg, err := getConfigAPIServer(c)
+	if err != nil {
+		if err := cli.ShowAppHelp(c); err != nil {
+			panic(err)
+		}
+		return nil, tracerr.Wrap(err)
+	}
+	return cfg, nil
+}
+
+func getConfigAPIServer(c *cli.Context) (*ConfigAPIServer, error) {
+	var cfg ConfigAPIServer
+	mode := c.String(flagMode)
+	nodeCfgPath := c.String(flagCfg)
+	var err error
+	switch mode {
+	case modeSync:
+		cfg.mode = node.ModeSynchronizer
+		cfg.server, err = config.LoadAPIServer(nodeCfgPath, false)
+		if err != nil {
+			return nil, tracerr.Wrap(err)
+		}
+	case modeCoord:
+		cfg.mode = node.ModeCoordinator
+		cfg.server, err = config.LoadAPIServer(nodeCfgPath, true)
 		if err != nil {
 			return nil, tracerr.Wrap(err)
 		}
@@ -398,6 +456,12 @@ func main() {
 			Usage:   "Run the hermez-node in the indicated mode",
 			Action:  cmdRun,
 		},
+		{
+			Name:    "serveapi",
+			Aliases: []string{},
+			Usage:   "Serve the API only",
+			Action:  cmdServeAPI,
+		},
 		{
 			Name:    "discard",
 			Aliases: []string{},