Make validator info as a historical data

For now validator-info dumps state of node structures into single file, so that we can just check node state at the time of the last dump.

It is very useful to have ability to get historical data of validator-info to track node's state transition in time.

Proposed solution:

• Add ability to save validator-info data in RocksDB storage in form
  • key = timestamp
  • value = json_data
• Add ability to switch between file dump (current implementation, just last state) and RocksDB (historical data)
• Change validator-info console app so that it checks config and finds out which data source to use. Several additional parameters may be added in case of RocksDB used, i.e. number of last records to return, time range etc.

Of course RocksDB with historical data is not for production usage.

Also for now there is no any synchronisation between validator-info writer and reader of file, it should be fixed too. Fortunately, RocksDB supports read-only mode, so there is no need to care about synchronisation here.