tombstone: Make ReversiblyMergeable
tombstone: Make ReversiblyMergeable

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