Additional BrokerConnection locks to synchronize protocol/IFR state

[dpkp]

@isamaru I threw this together as perhaps a middle-ground between the attempts to fix protocol-out-of-sync issues discussed in #1744 .

One small detail re: python atomic operations / locking: my understanding is that a simple boolean check on a python dict is atomic and would not require additional locks. Note the single CALL_FUNCTION opcode:

>>> import dis
>>> n = {'a': 1, 'b': 2}
>>> def foo():
...     bool(n)
...
>>> dis.dis(foo)
  2           0 LOAD_GLOBAL              0 (bool)
              2 LOAD_GLOBAL              1 (n)
              4 CALL_FUNCTION            1
              6 POP_TOP
              8 LOAD_CONST               0 (None)
             10 RETURN_VALUE

Of course, the value could still change between when the dict is loaded and when it is returned, which means the return value could be stale. But the same is true if we grab the lock, check the value, and then release the lock. As soon as we've released the lock, the value may become stale before we make our next decision. Which means that for the most part, if the value is important for decisions about state, we should check the value directly while holding the lock at the decision point. Wrapping an accessor method in a lock doesn't go far enough.

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This change is 

[dpkp]

There are still some issues related to the state_change_callback that need to be cleaned up, but I think they can be dealt with in separate PRs.

[isamaru]

@dpkp Thanks a lot, this looks better than my attempts. Will try 1.4.6.!