Highlight: Release for Bugfix and Code Refactor.

Feature: Retained message expiration (#182)

Improve: Add more metrics and ignore ‘$SYS/#’ publish (#266)

Improve: emqttd_sm should be optimized for clustered nodes may be crashed (#282)

Task: benchmark for 0.12.0 release (#225)

Benchmark: About 900K concurrent connections established on a 20Core, 32G CentOS server.