Now when the snapshot stopping is correctly handled, we may pull the database
reference all the way down to the schema::describe().
One tricky place is in table::napshot() -- the local db reference is pulled
through an smp::submit_to call, but thanks to the shard checks in the place
where it is needed the db is still "local"
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>