Highlight: Release for Bugfix and Code Refactor.
Feature: Retained message expiration (#182)
Improve: '$SYS/#' publish will not match '#' or '+/#' (#68)
Improve: Add more metrics and ignore '$SYS/#' publish (#266)
Improve: emqttd_sm should be optimized for clustered nodes may be crashed (#282)
Improve: Refactor emqttd_sysmon and suppress 'monitor' messages (#328)
Task: benchmark for 0.12.0 release (#225)
Benchmark: About 900K concurrent connections established on a 20Core, 32G CentOS server.