cdc: count touched mutation parts in transformer::transform
cdc: count touched mutation parts in transformer::transform

Modifies the transformer::transform so that it also returns a set of
flags indicating what parts of the mutation (e.g. rows, tombstones,
collections, etc.) were processed during transforming.

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