mutation_reader: move inactive_shard_reader code up
mutation_reader: move inactive_shard_reader code up

It will be used by the `evictable_reader` code too in the next patches.

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