Combine listenmux and streamrpc into a publicly exposed test RPC

Split off from #1125 (closed).

Outcome:

1. Add a new RPC TestStream to gitaly/proto, which uses the StreamRPC protocol
2. Put all the pieces together so that Gitaly can accept StreamRPC calls and handle the TestStream RPC over TCP, TLS, Unix sockets
3. The first iteration should include metrics, logging and authentication