Closes #602.
Also-by: N陳傑夫 <chienfuchen32@gmail.com>

...instead of just passing the value to the _published
condition.
Signed-off-by: NNick <nick.mayerhofer@enchant.at>

In order to enable waiting with timeout after issuing
  mqtt.Client's publish(..) like:
'''
infot = self.mqttc.publish(topic, msg, qos=1)
infot.wait_for_publish(0.5)
'''

Closes #555. Thanks to Charles Smith.

Fix identation and None comparation

Closes #500. Closes #89.

Closes #520. Thanks to Chris Hibberd.

Closes #566. Thanks to Cody Scott.

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.