Add cowboy_tracer_h stream handler

Another experimental stream handler. It enables tracing for
the connection process and any children processes based on
the matching of the request. It can be used to do ad-hoc
tracing by sending a specific header, path, method or other.

It is meant to be used both for tests and production. Some
configuration scenarios are NOT safe for production, beware.

It's important to understand that, at this time, tracing
is enabled on the scale of the entire connection including
any future request processes. Keep this in mind when trying
to use it in production. The only way to stop tracing is
by having the callback function exit (by calling exit/1
explicitly). This can be done after a certain number of
events for example. Tracing can generate a lot of events,
so it's a good idea to stop after a small number of events
(between 1000 and 10000 should be good) and to avoid tracing
the whole world.

Documentation will follow at a later time.

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