VPP-287: Fix bug in stop_servers
VPP-287: Fix bug in stop_servers

`CarrotRpc::ServerRunner#stop_servers` calls `s.queue.name`, but `s` is
a `CarrotRpc::RpcServer` and they don't have `#queue`, instead they have
`#server_queue`, so call that instead.

This bug was revealed by using RSpec 3 `instance_double` instead of
`double`; `instance_double` ensures the allowed or expected messages
are actually implemented by the class, which `double` does not.  We
shoudl be not mocking or only using `instance_double` to prevent similar
false positive tests.

C-S-D/carrot_rpc,C-S-D/carrot_rpc