Make public method for flushing pendingWrites

[rorShaK03]

When we call mutation operations on entites, method pendingExecute puts them in pendingWrites and merges and executes them on transaction end. Sometimes, it leads to so big query, which is obtained as a merge result, that DB couldn't process it.

In our case we can't use immediateWrites, because of performance and we cant make separate transactions because of data consistency.

As it seems to me, there to ways to fix:

1. (Simple) Make method flushPendingWrites public, so we can flush buffer time to time
2. (Harder) Flush buffer automatically when it become too big (in bytes)