Enhancements

Add HTTP API for hot configuration.

Bugfix

1. Hot configuration error of 'auth.mysql.password_hash' (emq-auth-mysql#68)

2. Hot configuration error of 'auth.pgsql.server' (emq-auth-pgsql#67)

3. Hot configuration error of 'auth.redis.server' and 'auth.redis.password_hash' (emq-auth-redis#47)

4. When deleting retained message subscribed clients not notified (emqttd#1207)