Split ExecStoreTuple into ExecStoreHeapTuple and ExecStoreBufferHeapTuple.

Upcoming changes introduce further types of tuple table slots, in
preparation of making table storage pluggable. New storage methods
will have different representation of tuples, therefore the slot
accessor should refer explicitly to heap tuples.

Instead of just renaming the functions, split it into one function
that accepts heap tuples not residing in buffers, and one accepting
ones in buffers.  Previously one function was used for both, but that
was a bit awkward already, and splitting will allow us to represent
slot types for tuples in buffers and normal memory separately.

This is split out from the patch introducing abstract slots, as this
largely consists out of mechanical changes.

Author: Ashutosh Bapat
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/20180220224318.gw4oe5jadhpmcdnm@alap3.anarazel.de

src/backend/executor/nodeSeqscan.c

@@ -84,15 +84,14 @@ SeqNext(SeqScanState *node)
 	 * our scan tuple slot and return the slot.  Note: we pass 'false' because
 	 * tuples returned by heap_getnext() are pointers onto disk pages and were
 	 * not created with palloc() and so should not be pfree()'d.  Note also
-	 * that ExecStoreTuple will increment the refcount of the buffer; the
+	 * that ExecStoreHeapTuple will increment the refcount of the buffer; the
 	 * refcount will not be dropped until the tuple table slot is cleared.
 	 */
 	if (tuple)
-		ExecStoreTuple(tuple,	/* tuple to store */
-					   slot,	/* slot to store in */
-					   scandesc->rs_cbuf,	/* buffer associated with this
-											 * tuple */
-					   false);	/* don't pfree this pointer */
+		ExecStoreBufferHeapTuple(tuple,	/* tuple to store */
+								 slot,	/* slot to store in */
+								 scandesc->rs_cbuf);	/* buffer associated
+														 * with this tuple */
 	else
 		ExecClearTuple(slot);