Fix a many-legged critter reported by chifungfan@yahoo.com: under the

Fix a many-legged critter reported by chifungfan@yahoo.com: under the
right circumstances a hash join executed as a DECLARE CURSOR/FETCH
query would crash the backend.  Problem as seen in current sources was
that the hash tables were stored in a context that was a child of
TransactionCommandContext, which got zapped at completion of the FETCH
command --- but cursor cleanup executed at COMMIT expected the tables
to still be valid.  I haven't chased down the details as seen in 7.0.*
but I'm sure it's the same general problem.