The current condition that consults the compaction manager
state for awaiting `_stop_future` works since _stop_future
is assigned after the state is set to `stopped`, but it is
incidental. What matters is that `_stop_future` is engaged.
While at it, exchange _stop_future with a ready future
so that stop() can be safely called multiple times.
And dropped the superfluous co_return.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit 6cce67bec8)
bdf31d7f54