rdbms: clean up DBO_TRX behavior for onTransaction* callbacks

rdbms: clean up DBO_TRX behavior for onTransaction* callbacks

* Make onTransactionIdle() wait until any transaction round
  is gone, even if there is no SQL transaction active. This
  is what onTransactionPreCommitOrIdle() already does.
* Decouple "transaction round mode" (DBO_TRX) from whether a
  round is active via a 'trxRoundId' LB info field. If rounds
  are enabled, but not is started, then the transaction state
  should be interpreted as "idle".
* Improve related documentation.
* Add more related unit tests.

Change-Id: I3ab18f577ec0375897fcb63f18f4ee2deeb436e9