messaging: don't inherit from seastar::rpc::protocol
messaging: don't inherit from seastar::rpc::protocol

messaging_service's rpc_protocol_server_wrapper inherits from
seastar::rpc::protocol::server as a way to avoid a
is unfortunate, as protocol.hh wasn't designed for inheritance, and
is not marked final.

Avoid this inheritance by hiding the class as a member. This causes
a lot of boilerplate code, which is unfortunate, but this random
inheritance is bad practice and should be avoided.

Closes #8084

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