multishard_combining_reader: move read-ahead related methods
multishard_combining_reader: move read-ahead related methods

To the group of methods that do not assume the reader is already
created. A patch will follow that will update read_ahead() to not assume
that the reader is created.

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