Optimize for Push Application, 500K+ Subscribers to a Topic.

Optimization for Route ETS insertion (#427)

Priority Message Queue for Persistent Session (#432)

Add Redis, MongoDB Plugins (#417)

Improve CLI Commands: pubsub, bridges, trace (#429)

emqttd_mod_subscription: fix client_connected/3

emqttd_auth_mod: add passwd_hash/2 function

priority_queue: add plen/2, out/2 functions

Add emqttd:seed_now/0 function

emqttd_plubin_mysql: Changed mysql driver to mysql-otp

emqttd_plugin_pgsql: Integrate with ecpool

emqttd_plugin_redis: First release