device: use channel close to shut down and drain outbound channel

device: use channel close to shut down and drain outbound channel

This is a similar treatment to the handling of the encryption
channel found a few commits ago: Use the closing of the channel
to manage goroutine lifetime and shutdown.
It is considerably simpler because there is only a single writer.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>