Concurrent, but not overlapping, MT test
Clariy TransactionService contract and update FDB impl
Bring PersistitTransactionService into line
Tighten up some FDB only test classes
merge nathanlws: Clarify TransactionService contract

Merge pull request #1286 from nathanlws/sql-layer/clarify-txn-service-contract
https://github.com/FoundationDB/sql-layer/pull/1286