Once unlink_leftmost_without_rebalance() has been called on a bi::set no
other method can be used. This includes clear_and_disposed() used by the
mutation_partition destructor.
We like unlink_leftmost_without_rebalance() because it is efficient, so
the solution is to manually finish destroying clustering row and range
tombstone sets in the reader destructor using that function.
Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
