Support for v3.1 sub protocol (Paho Library)

[francois-normandin]

The Paho library for Websockets uses a subprotocol version for distinction between 3.1 and 5.0 of MQTT.
As such, the CONNECT packet's variable header can have a different protocol name (0x00, 0x06 instead of 0x00-0x04).

Verify requirement change to support subprotocol. MQTT spec mentions that server MAY support subprotocols if desired. To be able to use the Paho WS library for javascript, this would be a nice-to-have in the control packets parsing algorithm.

http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718083

[francois-normandin]

Turns out the Paho library still supports 3.1 where CONNECT packet was 2 byte longer with protocol name MQIsdp.

Since the CONNECT Packet assumes a fixed length variable header of 10 bytes, the 12 byte packet is rejected as a protocol violation.

Solution to support different header in CONNECT packet is simple but need to go through the previous protocol to see if there are any incompatibilities.
ref: http://stanford-clark.com/MQTT/#connect