Introduce partition level atomicity for inserts/upserts/deletes on standalone collections

As mentioned in the standalone collections design doc the insert/upsert/delete statements on standalone collections need to be completely atomic.  

To accomplish this task, we will first introduce a change that makes insert/upsert/delete atomic on a single partition i.e., either all the changes made by theses statements corresponding to a single partition will be made durable or none.