Make sure the compaction_state:s are idle before
they are destroyed. Although all tasks are stopped
in stop_ongoing_compactions, make sure there is
fiber holding the compaction_state gate.
compaction_manager::remove now needs to close the
compaction_state gate and to stop_ongoing_compactions
only if the gate is not closed yet.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>