Split xml ContentHandler and OpenLCBNetwork

[Poikilos]

The new OpenLCBNetwork class (formerly named Dispatcher) is trimmed down after fixing #62, so it needs to be decided whether it is useful.

• PhysicalLayer/subclasses now have a _send_frames deque to let application do socket handling instead of needing OpenLCBNetwork to mitigate 2-way communication race conditions (pollState and pollFrame eliminate that) or to manage core states (Now CanLink does that).

• Clarify its responsibilities.

  • For example, it could still be used as a single class which instantiates the python-openlcb network stack.
    • Maybe keep ContentHandler superclass, but only use it when a MemoryRead that expects XML is being used (Make a high-level async readMemory method with a progress callback?)
  • Or eliminate it