Refactor/Misc Cleanup

FTP: Now tracks num_clients where it wasn't before
FTP: The stats page now uses port 38021 as the default
SFTP: The stats page now uses port 38022 as the default
General: Refactored known_fields out of  MetricCollector. This made
         the usage much cleaner
General: MetricCollector now implements start() and stop() to mimic other
         log handlers
General: num_clients is no longer an edge case for MetricCollector. E.G.
         log.msg(metric='num_clients')
         log.msg(metric='num_clients', count=-1)