Clean up a couple of problems in crosstab_hash's use of a hash table.

The original coding leaked memory (at least 8K per crosstab_hash call)
because it allowed the hash table to be allocated as a child of
TopMemoryContext and then never freed it.  Fix that by putting the
hash table under per_query_ctx, instead.  Also get rid of use
of a static variable to point to the hash table.  Aside from being
ugly, that would actively do the wrong thing in the case of re-entrant
calls to crosstab_hash, which are at least theoretically possible
since it was expecting the static variable to stay valid across
a SPI_execute call.