UnreadyService ignores dropped cancel handles

[jvff]

Motivation

The UnreadyService allows the request in flight to be cancelled through a oneshot channel. However, if the cancel handle (the oneshot::Sender endpoint) is dropped, the request isn't cancelled. This can lead to bugs and requests being kept executing when they should have been cancelled.

Alternatives

1. It might make sense to replace the "cancel handle" with some sort of LiveGuard type, that automatically signals that the request should be cancelled in its Drop implementation.

2. We could also check for Err(Canceled) in the existing code:
   

   zebra/zebra-network/src/peer_set/unready_service.rs

   Line 40 in b1acdc6

   if let Poll::Ready(Ok(CancelClientWork)) = this.cancel.poll(cx) {

Related Work

Teor uncovered this issue while reviewing #3108.

[mpguerra]

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