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.

db/migrations/0001.sql

@@ -661,6 +661,16 @@ CREATE TABLE account_creation_auth (
     timestamp TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT timezone('utc', now())
 );
 
+CREATE TABLE node_info (
+    item_id SERIAL PRIMARY KEY,
+    state BYTEA,            -- object returned by GET /state
+    config BYTEA,           -- Node config
+    -- max_pool_txs BIGINT,    -- L2DB config
+    -- min_fee NUMERIC,        -- L2DB config
+    constants BYTEA         -- info of the network that is constant
+);
+INSERT INTO node_info(item_id) VALUES (1); -- Always have a single row that we will update
+
 -- +migrate Down
 -- triggers
 DROP TRIGGER IF EXISTS trigger_token_usd_update ON token;
@@ -675,6 +685,7 @@ DROP FUNCTION IF EXISTS set_tx;
 DROP FUNCTION IF EXISTS forge_l1_user_txs;
 DROP FUNCTION IF EXISTS set_pool_tx;
 -- drop tables IF EXISTS
+DROP TABLE IF EXISTS node_info;
 DROP TABLE IF EXISTS account_creation_auth;
 DROP TABLE IF EXISTS tx_pool;
 DROP TABLE IF EXISTS auction_vars;