Praefect: add sidechannel support
This MR puts the pieces together for sidechannel support in Praefect.
- Accept
backchannel
connections from Gitaly clients - Proxy sidechannels from Gitaly clients to backend Gitaly servers via gRPC middleware
This completes gitlab-com/gl-infra/scalability#1218 (closed) and is part of gitlab-com/gl-infra&463 (closed).
Edited by Jacob Vosmaer