The original process is stopping etcd only when pipeline message finds itself
has been removed. After this PR, stream dial has this functionality too.
It helps fast etcd stop, which doesn't need to wait for stream break to
fall back to pipeline, and wait for election timeout to send out message
to detect self removal.
The messages in channel are outdated, and there is no need to send
them in the future. It also reports unreachable if there are messages
in the channel.
