I recently fixed a memory leak in an application where tokio's RawTask
storage was being kept alive by a leaked Waker. The task itself was
polling an `mpsc::Receiver` before then being aborted. During cleanup,
all references to the RawTask were dropped, except for the one stored in
`mpsc::chan::Chan::rx_waker`. While the `Receiver` was dropped as part
of the task's future, one of the channel's `Sender`s was leaked outside
the task. This meant the `Chan` was never dropped and its `rx_waker`
contained the leaked Waker.
I fixed the leak by properly cleaning up the `Sender`, but I also think
keeping `rx_waker` around in this case is unnecessary. Once `chan::Rx`
is dropped, it can't be polled anymore, so waking up the registered task
will always be spurious.
The commit includes a regression test to illustrate the problem that is
fixed by removing the waker explicitly.
