Still to do:
- consuming loop not done
- figure out if we can do consuming loop somehow in the actor thread or
we need to spanw thread ( and change request sender not to receive self)

- still missing free http connection counters

priority is type specific, not instance specific and therefore should
not have &self in the signature of WithRequestPriority::priority()

Changed for interface legibility of.

Instead of using an mpsc channel to notify the end of a tracked future
now we use an `Recipient<TrackedFutureFinished>` with the handler
implemented in `UpstreamRelay`