We'd like to use compaction_state::gate both for functions
running with compaction disabled and for and tasks referring
to the compaction_state so that stop_ongoing_compactions
could wait on all functions referring to the state structure.
This is also cleaner with respect to not relying on
gate::use_count() when re-submitting regular compaction
when compaction is re-enabled.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>