Aggregate changes from multiple transactions in AsyncStore

After debugging ISPN-618 we found out that the different changes of different transactions are not being coalesced in a smaller set of changes in the AsyncStore (see mailing list).
The AsyncStore implementation should aggregate changes in a "transactions" variable in the same way as AbstractCacheStore, so to push only committed Modifications to the state-changes which are going to be async-applied to the delegated store.

So it seems to make sense to have AbstractDelegatingStore extend the AbstractCacheStore to inherit the same prepare/commit/rollback methods, but this affects all implementations of AbstractDelegatingStore.