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/nodeIndexonlyscan.c

@@ -34,6 +34,7 @@
 #include "executor/execdebug.h"
 #include "executor/nodeIndexonlyscan.h"
 #include "executor/nodeIndexscan.h"
+#include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/predicate.h"
 #include "utils/memutils.h"
@@ -117,6 +118,8 @@ IndexOnlyNext(IndexOnlyScanState *node)
 	{
 		HeapTuple	tuple = NULL;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * We can skip the heap fetch if the TID references a heap page on
 		 * which all tuples are known visible to everybody.  In any case,