Generalize Gitaly backchannel to allow connection hijacking

Separate Gitaly's backchannel package into two parts:

1. gRPC listener multiplexing
2. Code specific to Praefect <-> Gitaly backchannel communication

Outcome: the code that plugs backchannel into the main Gitaly server is changed so that it inserts the listener multiplexer, and registers backchannel as a multiplexer target.

Background discussion can be found in #1051 (comment 567753158).

Status 2021-06-16

Submitted a cleanup MR for the code I'll be working on gitlab-org/gitaly!3591 (merged).

Assigned main MR for first review gitlab-org/gitaly!3593 (merged).