Send queue has a size limit of 65555 when using QoS > 0

Contrary to the documentation, which states that there is an unlimited queue size by default, the send queue for messages with QoS > 0 is limited to 65555 entries via this `if` statement:
https://github.com/eclipse/paho.mqtt.python/blob/master/src/paho/mqtt/client.py#L1210

I think the code could be changed so that message ids are assigned later, when the message should actually be sent and a message id is available again.