mod_cloud_notify: use stanza from mod_smacks to behave O(1) This uses the provided stanza in the ack-delayed smacks event to trigger push for this stanza only rather than iterating through the whole smacks queue to find the first unpushed stanza.

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.