Make GIN's cleanup pending list process interruptable

Cleanup process could be called by ordinary insert/update and could take a lot
of time. Add vacuum_delay_point() to make this process interruptable. Under
vacuum this call will also throttle a vacuum process to decrease system load,
called from insert/update it will not throttle, and that reduces a latency.

Backpatch for all supported branches.

Jeff Janes <jeff.janes@gmail.com>

src/backend/access/gin/ginfast.c

@@ -810,8 +810,7 @@ ginInsertCleanup(GinState *ginstate,
 		 */
 		processPendingPage(&accum, &datums, page, FirstOffsetNumber);
 
-		if (vac_delay)
-			vacuum_delay_point();
+		vacuum_delay_point();
 
 		/*
 		 * Is it time to flush memory to disk?	Flush if we are at the end of
@@ -851,8 +850,7 @@ ginInsertCleanup(GinState *ginstate,
 			{
 				ginEntryInsert(ginstate, attnum, key, category,
 							   list, nlist, NULL);
-				if (vac_delay)
-					vacuum_delay_point();
+				vacuum_delay_point();
 			}
 
 			/*
@@ -937,7 +935,7 @@ ginInsertCleanup(GinState *ginstate,
 		/*
 		 * Read next page in pending list
 		 */
-		CHECK_FOR_INTERRUPTS();
+		vacuum_delay_point();
 		buffer = ReadBuffer(index, blkno);
 		LockBuffer(buffer, GIN_SHARE);
 		page = BufferGetPage(buffer);