R1:08b1a33dc36a

Implement a Git LFS server which supports no operations

Summary:
Ref T7789. This builds on top of `git-lfs-authenticate` to detect LFS requests, read LFS tokens, and route them to a handler which can do useful things.

This handler promptly drops them on the floor with an error message.

Test Plan:
Here's a transcript showing the parts working together so far:

- `git-lfs` connects to the server with SSH, and gets told how to connect with HTTP to do uploads.
- `git-lfs` uses HTTP, and authenticates…