Allow interrupting GetMultiXactIdMembers

This function has a loop which can lead to uninterruptible process
"stalls" (actually infinite loops) when some bugs are triggered.  Avoid
that unpleasant situation by adding a check for interrupts in a place
that shouldn't degrade performance in the normal case.

Backpatch to 9.3.  Older branches have an identical loop here, but the
aforementioned bugs are only a problem starting in 9.3 so there doesn't
seem to be any point in backpatching any further.

src/backend/access/transam/multixact.c

@@ -1278,6 +1278,7 @@ retry:
 		{
 			/* Corner case 2: next multixact is still being filled in */
 			LWLockRelease(MultiXactOffsetControlLock);
+			CHECK_FOR_INTERRUPTS();
 			pg_usleep(1000L);
 			goto retry;
 		}