mutation_source: Cleanup
mutation_source: Cleanup

 - combines telescopic overloads into one method with default paramters.
 - Introduce func_type for a full handler to avoid some duplication.

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