Gmqtt

Bug fixes in async `on_message` callback. Now it works properly for partial coroutines and `optimistic_acknowledgement=true` clients

* fixed disconnect packet in MQTT3.1.1 This bug caused unexpected will messages coming after gentle disconnect
* added info about asynchronous `on_message` callback to README

* Multiple topics subscription added:

client.subscribe([gmqtt.Subscription('TEST/+', qos=1), gmqtt.Subscription('TEST', qos=0)],
subscription_identifier=1)

* Fixed resend `QoS > 0` messages
* Added `Client.is_connected` property, which tells if connection is alive and CONNACK was received

Removed unnecessary link to current event loop in sync method and etc.

Optimized packets read and parse

* Added support for asynchronous callback messages. Value returned from callback future will be used as reason code in `PUBACK` packet
* added method to update client's `_reconnect` flag