mod_firewall: Initialize compiled chunk just once for all handlers This should fix a case where some stateful dependencies (such as throttles) produce separate instances for every call to new_handler(), leading to surprising behaviour (e.g. rules executed via JUMP CHAIN vs ::deliver would have separate rate limits). This also adds better error handling in case the compiled code fails to run for some reason.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).