hedge: don't reserve slots for hedged requests

[hdevalence]

Hedge wraps S: Service + Clone and is essentially implemented as
Select<S, Delay<S>>, whose call implementation creates two response
futures and selects the one which resolves first. The second response
future is obtained from Delay<S>::call, which waits for some time
before calling S::call.

The current implementation of Delay<S>::poll_ready calls
S::poll_ready unconditionally. However, the S::call corresponding
to that poll_ready only happens after much delay, if ever. This
interacts badly with Buffer, which uses poll_ready to reserve a
channel slot. When using Hedge with a Buffer'd service, the inner
Buffer::poll_ready is called twice, but only one slot is used
immediately. This causes exhaustion of all of the Buffer slots.

Instead, return Poll::Ready(Ok(())) from Delay::poll_ready and use a
Oneshot inside of the Delay future.

[hawkw]

Just to confirm, this fixes an issue in Zebra, correct?

[hdevalence]

Yes, or it would, if the code that uses it was merged. The Zebra context is here: ZcashFoundation/zebra#1089

[hawkw]

this seems right to me!