pgq/triggers: Fix crash in case invalidate happens from signal handler.

pgq/triggers: Fix crash in case invalidate happens from signal handler.

Currently code assumed the invalidate happens only from per-query checks,
but this seems not to be the case.

Fix it by moving reset code out from invalidate callback.

Also old code seemed to leak htab per full reset, because it assumed
to be located under cache context, but init code did not assign it there.

Thanks to Andrew Dunstan for the report.