Replace callbacks with event dispatcher in UnleashClient and its Connectors

[ppicom]

Summary

This pull request wires the EventDispatcher to the UnleashClient and its connectors. It changes all connectors from expecting an event_callback
(and, in some instances, a ready_callback) to accepting an instance
of the newly created EventDispatcher.

Tricky things when reviewing

unleash_event_callback instance variable

After reviewing #243, it seems like that property was not meant to be public.
Furthermore, it is not mentioned in any docs. However, it would be a breaking
change to remove it, and rude to any users who might be using it.

I'd clean it up otherwise.

events.py utilities

Testing a dispatcher of events that uses Threads is hard. I've introduced
an EventRecorder helper class that aids in testing that the publishing
of events works as intended without having to poke the internals of
the class being tested.

I agree, though, that it can be funny. I'm happy to review this decision if
better alternatives come to mind.

asserting awaited signals

Because the callback ran in the same thread as tests, ensuring signals
were awaited instead of timed out made sense:

    ready_signal.wait(timeout=1) # We don't care about the return value

Callbacks run in another thread now, so asserting these waits makes
more sense:

    assert ready_signal.wait(timeout=WAIT_TIMEOUT)

BoostrapConnector behaves funny

Two things worth noting about this connector:

1. It does not emit events because it never accepted a callback in the
   first place.
2. It's the only connector which start() function is called in the
   UnleashClient constructor.