raft: replication test: move structs into class
raft: replication test: move structs into class

Move auxiliary classes connection and hash_connection out of
raft_cluster and into connected class.

Signed-off-by: Alejo Sanchez <004be06a28e38b437b038f9b5c0b9f5758b792e9@scylladb.com>

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