Use embedded Go stubs for gitaly-proto

This is another step towards #1761 (closed)

Proto changes must still be made via gitlab-org/gitaly-proto, but when you pull them into gitaly, we actually vendor the .proto files and re-generate the stubs.

The diff is very large because this updates all the import paths.

Asides from a change in import path, this should cause NO change in workflow for us yet.