Run a portal's cleanup hook immediately when pushing it to DONE state.

This works around the problem noted by Yamamoto Takashi in bug #5906,
that there were code paths whereby we could reach AtCleanup_Portals
with a portal's cleanup hook still unexecuted.  The changes I made
a few days ago were intended to prevent that from happening, and
I think that on balance it's still a good thing to avoid, so I don't
want to remove the Assert in AtCleanup_Portals.  Hence do this instead.

src/backend/tcop/pquery.c

@@ -814,7 +814,7 @@ PortalRun(Portal portal, long count, bool isTopLevel,
 							   dest, altdest, completionTag);
 
 				/* Prevent portal's commands from being re-executed */
-				portal->status = PORTAL_DONE;
+				MarkPortalDone(portal);
 
 				/* Always complete at end of RunMulti */
 				result = true;