Parallelize github event processing

The GitHub driver does an event pre processing before adding a trigger
event to the scheduler. This is currently done single threaded to
ensure that the trigger events are enqueued in order into the
scheduler. A problem is that this pre processing can take a few
seconds which limits the rate of events we can process. In order to
parallelize this while keeping the order of the trigger events we need
to do two things.

We keep consuming the event queue single threaded but instead of
processing and forwarding them directly to the scheduler we process
them in a thread pool and put the futures into a result queue. This
second queue can then again be processed single threaded and maintains
the correct ordering of the events.

Second updating the change cache currently assumes that it runs single
threaded. In order to avoid data races we need to lock this by the
change.

Change-Id: I08f31b99e2a58e51cef0de89edd98c957d7db87f

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