Support $SYS Topics of Broker Now!
Feature: emqtt_broker to publish version, uptime, datetime to $SYS/brokers/# topics
Feature: emqtt_broker to publish count of clients, sessions, suscribers to $SYS/brokers/# topics
Feature: emqtt_metrics to publish bytes, packets, messages metrics to $SYS/brokers/# topics
Feature: add include/emqtt_systop.hrl
Change: emqtt_cm to count current clients
Change: emqtt_sm to count current sessions
Change: emqtt_pubsub to count current topics and suscribers
Change: emqtt_pubsub to add create/1 API
Change: emqtt_pubsub dispatch/2 to return number of subscribers
Change: emqtt_pubsub to count 'dropped' messages
Change: emqtt_opts to add merge/2 function
Test: add emqtt_serialiser_tests.erl