For flushing 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 flush` translates to
a sequence of `/storage_service/keyspace_flush` calls).
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
(cherry picked from commit 1b576f358b)