Optimize session management and upgrade eSockd library to 2.7.1
Improve: issue#294 - [error] failed to start connection on 0.0.0.0:1883 - enotconn
Improve: issue#297 - How do I allow user with some pattern to access topic with some pattern?
Bugfix: issue#291 - "./bin/emqttd attach ..." cannot work
Bugfix: issue#284 - Should not use erlang:list_to_atom/1 in emqttd_vm.erl