range_tombstone: add flip_bound_kind()
range_tombstone: add flip_bound_kind()

flip_bound_kind() changes start bound to end bound and vice versa while
preserving the inclusivness.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla