Make send of DISCONNECT packet optional on disconnect()

[fpagliughi]

A while back there was an issue on the Paho C++ repo requesting that client not send a DISCONNECT when it gets destroyed:

eclipse/paho.mqtt.cpp#372

I guess since, technically, the sending of the DISCONNECT packet is optional according to the spec, the sending of the DISCONNECT packet could be made optional in the libary, right? Perhaps add a boolean field to the MQTTAsync_disconnectOptions struct, like int sendDisconnect which would default to 1 (true), but could be set 0/false by the user.

Then, the library would just shut down the connection without sending the DISCONNECT packet.