atomic_cell_hash: Specialize appending_hash<> for atomic_cell and collection_mutation
atomic_cell_hash: Specialize appending_hash<> for atomic_cell and collection_mutation

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