MODE-1854 - Changed the way indexing is performed when user transactions are active

Up until now, tasks were submitted to Hibernate Search only when the transaction had the STATUS_COMMITTED. This, in turn, caused Hibernate Search to not see the indexing changes as part of a running transaction and ran each of the changes "out-of-band".

The change was to effectively use a "standard monitor" (instead of an accumulating monitor) which will send any index changes to Hibernate Search directly after they occur (in _Session#persistChanges_). 

Since we're also giving Hibernate Search the transactional context (with an active transaction), it will enqueue the changes and run them later on, via a _Synchronization_.