DRILL-2178: Update outgoing record batch size and allocation in PartitionSender.
Also:
+ Add setInitialCapacity() method to ValueVector interface set the initial capacity
of memory allocated in first allocateNew() call.
+ Send an empty batch for fast schema instead of flushing the OutgoingRecordBatches
which throw away allocated memory and reallocate again.
+ Remove the v.getValueVector().makeTransferPair(outgoingVector) hack as the complex
schema child schema population bug is fixed in DRILL-1885.
+ Cleanup/refactor PartitionSender related code.
