Allow hash joins to be interrupted while searching hash table for match.

Allow hash joins to be interrupted while searching hash table for match.

Per experimentation with a recent example, in which unreasonable amounts
of time could elapse before the backend would respond to a query-cancel.

This might be something to back-patch, but the patch doesn't apply cleanly
because this code was rewritten for 9.1.  Given the lack of field
complaints I won't bother for now.

Cédric Villemain