mod_storage_xmlarchive: Fix return of numeric 'when' from key-value API At some point, 'when' was stored as a timestamp string then as a numeric UNIX timestamp, so both cases needs to be handled, as they are elsewhere. Thanks spec/core_storagemanager_spec.lua

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.