mutation_reader: refactor prepare_next
mutation_reader: refactor prepare_next

Move out logic responsible for adding readers at partition boundary
into `maybe_add_readers_at_partition_boundary`, and advancing one reader
into `prepare_one`. This will allow to reuse this logic outside
`prepare_next`.

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