If the procedure's control code marks a SQLStmt batch as the last one that it will submit, we will notify all partitions that we are done with them. For now we only do this if the last batched is non-local, but it should be easy to hook it in when a dtxn dispatches a single-partition/local batch. Refactored the PartitionExecutor a bit to remove some permananent global objects (to reduce GC overhead) and to make PartitionExecutor.computeDonePartitions() easier to understand