mod_muc_log_http: use datamanager:stores() w/ 'pattern' parameter, use datamanger.store_exists()

There's still a call to lfs in the day_callback function that looks like it could be replaced with store_exists (https://github.com/jprjr/metronome/blob/72d8b29372a8afefbaf98636c7f82388e5fe6304/plugins/muc_log_http/mod_muc_log_http.lua#L190-Lundefined). It just needs the host passed as a parameter.