Implement query service

[rekby]

With two methods:

1. Query("query text", query_params, optional transaction mode, lambda (result stream))
2. QueryTx(optional transaction mode, lambda(tx))

lambda is retriable user callback. When use queryTx - transaction will commit by default and has way for rollback (exception + tx.Rollback()).

And see to spanner.

Roadmap:

• [X] Implement .Query() - method for single SQL execution without transaction creation
• [ ] Implement .QueryTx() method for multiple SQL executions on same transaction waiting for Begin/Commit/Rollback transaction methods will be realized on server (at the moment transactions works using TableService methods)
• [ ] Session attach/detach
• [X] Merge implementation of SessionPools from Query and Table services
  • [X] Attach
  • [x] Detach
• [X] Retryer
• [X] Examples
• [ ] helper methods such as Select, SelectFirstOrDefault, NonQuery etc.