Add HTTP API for hot configuration.

Bugfix

1. Parse ‘auth.mysql.password_hash’ error when hot configuration reload (emq-auth-mysql#68)
2. Set ‘auth.redis.server’ and ‘auth.redis.password_hash’ error when hot configuration reload (emq-auth-redis#47)
3. Fix the issue that when deleting retained message subscribed clients are not notified (emqttd#1207)
4. Support more parameters for hot configuration reload:

• mqtt.mqueue.low_watermark = 20%
• mqtt.mqueue.high_watermark = 60%
• mqtt.client.idle_timeout = 30s