mod_pubsub_serverinfo: Warm-up opt-in cache By warming up the cache that contains the opt-in data, the first publication has a better chance of including domain names for remote domains that opt-in. Without this change, those domains are named only after the _second_ publication, which can take a while. New users are likely thrown off by that.

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.