Use async backpressure instead of MaxSlotsExceeded error

[nazar-pc]

Currently library has an annoying behavior where exceeding max number of requests returns an error immediately.

I would have expected for it to be delayed until request can be made and user can cancel request before it goes out. Current behavior would be something like try_request() instead.

This can be achieved with a semaphore internally before trying to do anything.

Without this capability we are forced to add semaphore on application level even though we have already configured number of concurrent requests on the client level.

[niklasad1]

Right,

I think we can adjust this limit to only configure the bounded channel instead for backpressure only and let user handle slow calls etc such as utilizing the timeouts.

I don't remember why I added that but backpressure makes more sense to me here and supporting try_request() may be an option but not sure.