Multiprocessing and version 1.2

I've noticed that in version 1.2 each mqtt message now has a thread lock associated with it, which means that it's impossible to add a message to a multiprocessing queue. The project I'm using this for has to use multiprocessing, so I've changed this in my own fork so that there just isn't a lock because I'm never accessing messages in a way that would cause a data race.

It would be better if there was some way to specify whether to use threading locks or multiprocessing locks for messages.