mod_http_muc_log: Correctly handle changed or retracted reactions Since per XEP-0444 each reaction should overwrite all previous reactions on a particular message from a particular occupant. Previously repeated reactions would be counted again and retractions were not handled.

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.