mod_s2s_keepalive: Isolate source host of pings The incoming_s2s table is not restricted to the current virtualhost so this prevents opening more connections than what's needed. Also prevents useless double sending of one whitespace per local host.

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.