atomic_cell: Add REVERT flag
atomic_cell: Add REVERT flag

Needed to make atomic cells ReversiblyMergeable.

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