This fixes a bug introduced when we added heartbeat support. Recall that we
handle the Bitcoin connection state machine on a per-peer basis. Each
connection has a task created from the `Connection` struct, and a `Client:
tower::Service` "frontend" that passes requests to it via a channel. In the
`Connection` event loop, the connection checks whether the request channel has
been closed, indicating no further requests from the `Client`, in which case it
shuts itself down and cleans up resources. This occurs when all of the senders
have been dropped.
However, this behavior broke when we introduced heartbeat support, because we
spawned an additional task to send heartbeat messages along the request
channel. This meant that instead of having a single sender, dropped by the
`Client`, we have two senders, the `Client` and the "shadow client" task that
generates heartbeat messages. This means that when the `Client` is dropped, we
still have a live sender and the connection is not closed. To fix this, the
`Client` now uses a `oneshot` to shut down its corresponding heartbeat task.
This closes all senders.
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