mod_rest: Workaround lack of number coercion in util.datamapper for HTTP upload util.datamapper will not coerce a string into an integer like the XEP-0363 'size' slot, which becomes a problem when using the HTTP GET method, passing fields as ?query parameters which are always strings.

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.