InFlightLimit middleware

[carllerche]

This PR adds an InFlightLimit middleware that enforces a max number if in-flight requests for the inner service as well as all clones.

This middleware works for both Service and ReadyService.

[carllerche]

The implementation should be ready to go. There is one remaining question around trait implementations.

InFlightLimit applies both to Service and ReadyService. Currently, InFlightLimit<T: ReadyService> implements ReadyService itself. However, it could also implement Service directly because the limiting factor is itself the poll_ready implementation.

The problem is, coherence doesn't let us implement both Service AND ReadyService for InFlightLimit<T: ReadyService> if there also is an implementation for T: Service...

One solution would be to have separate middleware for taking a T: ReadyService and lifting it to a Service by bounding the max in-flight.

[carllerche]

Great, I'm going to merge the PR as is. Handling the issue discussed in the above comment depends on the fate of ReadyService (#50).