DRILL-2187: Single Broadcast Sender
Also includes:
1. Fix merge join planning issue (1c5c810 by jinfengni)
2. ExternalSort: Check the memory available for in-memory sorting or not in making decision to spill or not (36f9dd1)
3. Cleanup in ExternalSortBatch and its helper classes (36f9dd1)
4. MergeJoinBatch: Limit the outgoing record batch size to 2^15 (37dfeb8)
5. StreamingAggBatch: Limit outgoing record batch size to 2^15 (7d8a2e4)
