Add the CompactedFragmentsConsumer
Add the CompactedFragmentsConsumer

Undust the commented CompactMutationConsumer concept, make it usable and
rename it to CompactedFragmentsConsumer (as we not have flat readers).

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