mod_smacks: fix bug for missbehaving clients sending multiple acks in a row Missbehaving clients, sending multiple acks in a row (I'm looking at you Monal!) triggered the ack-loop-prevention code added in 2017, leaving unacked stanzas in the queue. This fixes the bug while still preventing ack-loops.

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.