pgsql: Marginal performance improvements in dynahash: make sure that

Log Message:
-----------
Marginal performance improvements in dynahash: make sure that everything
associated with a hashtable is allocated in that hashtable's private
context, so that hash_destroy only has to destroy the context and not
do any retail pfree's; and tighten the inner loop of hash_seq_search.

Modified Files:
--------------
    pgsql/src/backend/utils/hash:
        dynahash.c (r1.58 -> r1.59)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/hash/dynahash.c.diff?r1=1.58&r2=1.59)