* refactor(emqx_slow_subs): refactor use moving average
* fix(emqx_slow_subs): change elapsed to latency, and fix some error
* fix(emqx_slow_subs): fix emqx_mgmt_api.erl indent
* fix(emqx_slow_subs): change api name
* fix(emqx_slow_subs): fix and improve some code
* fix(emqx_slow_subs): move clienid filed from latency_stats to session
lafirest