Constify the connection IDs used by internal services.

All connection IDs used by internal adapters are now grouped in ClientInterface
as constants.
ENG-5040: Add tokens accessor to ElasticHashinator.
Remove legacy code from TransactionCreator.

The methods were used in legacy mode in RestoreAgent. No longer needed.
ENG-5040: Call requestIndexSnapshot before command log replay.
Merge pull request #730 from VoltDB/ENG-5040

ENG-5040: Index snapshot initiation on recover and various improvements