mod_client_management: Report on longest lived token when grant does not expire E.g. for mod_http_oauth2 where by default the grant itself is unlimited, while refresh tokens are issued with one week lifetime, but are renewed with each use.

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.