- 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.

- Add L1Tx TxCompressedData method
- Add PoolL2Tx TxCompressedDataV2 method
- Update ProcessTxs logic
- Add ZKInputs Intermediate States & Fee parameters calculation

Update log package with fields & file log, and update constants in the
repository code to be MixedCase.

resolves #91, resolves #92, resolves #95

- gofmt - Gofmt checks whether code was gofmt-ed. By default this tool runs
  with -s option to check for code simplification
- goimports - Goimports does everything that gofmt does. Additionally it checks
  unused imports
- golint - Golint differs from gofmt. Gofmt reformats Go source code, whereas
  golint prints out style mistakes
    - checks the uncommented exported functions & types

Update the code to fix the lint checks.

Add linter checks to GHA & Fix code to pass lints.
The linters added are:
- whitespace: Tool for detection of leading and trailing whitespace
- gosec: Inspects source code for security problems
- gci: Gci control golang package import order and make it always deterministic
- misspell: Finds commonly misspelled English words in comments
- gomnd: An analyzer to detect magic numbers

The file utils/utils.go is excluded from the checks of gomnd, as uses magic numbers through the code