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 writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.