DATAMONGO-737 - Register TransactionSynchronization holder once per Mong...

Pull Request for DATAMONGO-737 fix. This change includes the fix and test cases simulating a transaction completion run against the Mongo TransactionSynchronizations registered in with the TransactionSynchronizationManager. The fix itself is straightforward: only register the TransactionSynchronization for a Mongo instance once, not for each database accessed within the transaction. Testing for this issue requires accessing more than one database within a transaction: this explains why it is not a common scenario that would have surfaced prior.