Change hashscan.c to keep its list of active hash index scans in

Change hashscan.c to keep its list of active hash index scans in
TopMemoryContext, rather than scattered through executor per-query contexts.
This poses no danger of memory leak since the ResourceOwner mechanism
guarantees release of no-longer-needed items.  It is needed because the
per-query context might already be released by the time we try to clean up
the hash scan list.  Report by ykhuang, diagnosis by Heikki.

Back-patch to 8.0, where the ResourceOwner-based cleanup was introduced.
The given test case does not fail before 8.2, probably because we rearranged
transaction abort processing somehow; but this coding is undoubtedly risky
so I'll patch 8.0 and 8.1 anyway.