For major compacting all tables in the database.
The advantage of this api is that `commitlog->force_new_active_segment`
happens only once in `database::flush_all_tables` rather than
once per keyspace (when `nodetool compact` translates to
a sequence of `/storage_service/keyspace_compaction` calls).
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit b12b142232)
3ff8051532