Data race when sending and receiving after closing a `oneshot` channel

If a tokio::sync::oneshot channel is closed (via the
oneshot::Receiver::close method), a data race may occur if the
oneshot::Sender::send method is called while the corresponding
oneshot::Receiver is awaited or calling try_recv.

When these methods are called concurrently on a closed channel, the two halves
of the channel can concurrently access a shared memory location, resulting in a
data race. This has been observed to cause memory corruption.

Note that the race only occurs when both halves of the channel are used
after the Receiver half has called close. Code where close is not used, or where the
Receiver is not awaited and try_recv is not called after calling close,
is not affected.

See tokio#4225 for more details.