Appending to bytes() is slow, bytearray() is fast.

Closes #571. Thanks to Umberto Morelli.

It would sometimes call the MQTT v3.x callback when it should
call the MQTT v5 callback.

Closes #570. Thanks to Frank Pagliughi.

Closes #575. Thanks to briggySmalls.

v5 support, nicer expect_packet() support, as from Mosquitto.

Fix None comparation as PEP

And convert the _out_packet queue use to thread safe.

Closes #586. Thanks to Sam McKelvie.

Merge branch 'external-loop-publish-qos0' of git://github.com/jeeger/paho.mqtt.python into external-loop-publish-qos0

Merge branch 'feat-reconnect-on-protocol-failure' of git://github.com/sbtinstruments/paho.mqtt.python into sbtinstruments-feat-reconnect-on-protocol-failure

Closes #409
Closes #576
Thanks to Thomas Zahari.

Added MQTTv5 version to README

Updated MQTT sandbox broker URL.

The `rc` parameter in the `on_disconnect` callback now has meaningful values
in the case of an error. Closes #441. Thanks to Nawab.