mod_pubsub_subscription: Match stanzas with fixed prefix only This should help filter out any other unrelated iq stanzas which may be exchanged with the pubsub service. For example, a separate module was requesting the item list from a subscribed node, but this module was incorrectly attempting to handle the result.

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.