sstables: mutation_reader: Move definitions out of the class body
sstables: mutation_reader: Move definitions out of the class body

To make further refactoring easier to review. No functional changes here.

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