crimson/osd: use abort_source in stop_signal

crimson/osd: use abort_source in stop_signal

before this change, `stop_signal::wait()` waits until it receives
`SIGTERM` or `SIGINT`, but we also need to stop the service per the
request of monitor or when a serious health condition is detected.

so, an `abort_source` should allow the server to request abort by
itself. also, as the single truth of stop, `stop_signal` will be
able to send the message to its subscribers to abort any "blocking"
calls which might prevent or delay the stop process.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>