storage_service: Move some code up the file
storage_service: Move some code up the file

No logic change, this is to keep join_token_ring next to
prepare_to_join so that the patch merging them becomes clean and small.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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