Currently, `Service` does not provide a mechanism by which it can signal
to the caller that it is at capacity. This commit adds a `poll_ready`
function to the `Service` trait. Callers are able to first check
`poll_ready` before calling `Service::call`.
`poll_ready` is expected to be a hint and will be implemented in a best
effort fashion. It is permitted for a `Service` to return `Ready` from
`poll_ready` and the next invocation of `Service::call` fails.
Carl Lerche