Move interrupt checking from ExecProcNode() to executor nodes.

In a followup commit ExecProcNode(), and especially the large switch
it contains, will largely be replaced by a function pointer directly
to the correct node. The node functions will then get invoked by a
thin inline function wrapper. To avoid having to include miscadmin.h
in headers - CHECK_FOR_INTERRUPTS() - move the interrupt checks into
the individual executor routines.

While looking through all executor nodes, I noticed a number of
arguably missing interrupt checks, add these too.

Author: Andres Freund, Tom Lane
Reviewed-By: Tom Lane
Discussion:
    https://postgr.es/m/22833.1490390175@sss.pgh.pa.us

src/backend/executor/nodeHashjoin.c

@@ -92,6 +92,14 @@ ExecHashJoin(HashJoinState *node)
 	 */
 	for (;;)
 	{
+		/*
+		 * It's possible to iterate this loop many times before returning a
+		 * tuple, in some pathological cases such as needing to move much of
+		 * the current batch to a later batch.  So let's check for interrupts
+		 * each time through.
+		 */
+		CHECK_FOR_INTERRUPTS();
+
 		switch (node->hj_JoinState)
 		{
 			case HJ_BUILD_HASHTABLE:
@@ -246,13 +254,6 @@ ExecHashJoin(HashJoinState *node)
 
 			case HJ_SCAN_BUCKET:
 
-				/*
-				 * We check for interrupts here because this corresponds to
-				 * where we'd fetch a row from a child plan node in other join
-				 * types.
-				 */
-				CHECK_FOR_INTERRUPTS();
-
 				/*
 				 * Scan the selected hash bucket for matches to current outer
 				 */