range_tombstone: add flip()
range_tombstone: add flip()

range_tombstone::flip() flips range bounds. This is necessary in order
to use range tombstone in reversed mutation fragment streams.

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

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