Substrate-BFT task cancelling

[rphmeier]

Dropping AgreementHandle waits for a Task to be sent over a channel, which happens when the BftFuture is first polled or on drop. However, this can race against the local thread's event loop being turned.

The simple fix is to alter AgreementHandle to store a oneshot::Sender<()> and BftFuture to store the receiver, and cancel itself upon hearing something on that channel.