Replace mutex in autopaho with atomic pointer

[thedevop]

Is your feature request related to a problem? Please describe.
Autopaho currently use mutex for Publish/Subscribe/etc. It can lead to lock contentions.

Describe the solution you'd like
Replace mutex with atomic.Pointer. Put cli, connUp, and connDown into one struct, so it can be stored in one atomic operation.

Additional context
Here is the implementation, only autopaho/auto* are modified.

[MattBrittan]

Can you please provide more information re "lock contentions", have you identified a performance issue in a real application which this change fixes?

I'm open to the use of atomic.Pointer but feel that mutex is generally easier to understand and more widely used. As such I have a preference for a mutex unless there is a fairly compelling reason to change (in line with the docs "Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package").

[thedevop]

The lock contentions stem from upper layer application, where there are many Go routines that Publish/Subscribe/Unsubscribe through the client. But with current Subscribe/Unsubscribe (and to certain extent Publish, as async Publish doesn't return broker ack) as blocking operations until ack from broker, funneling them through a single channel is not viable.

[MattBrittan]

And has the change to atomic.Pointer made a measurable difference? The locks on ConnectionManager.Mu should be very brief because it's only locked for the time taken to grab a copy of cli. async Publish is the subject of issue #216 and this is complicated by the queue and store (I have made a suggestion as to one way to address this which would be fairly easy to implement but looking for some feedback).