cdc: move management of generated mutations inside transformer
cdc: move management of generated mutations inside transformer

CDC log mutations are now stored inside `transformer`, and only moved to
the final set of mutations at the end of `transformer`'s lifetime.

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla