* go mod init; rm -rf vendor
* tweak proto files and generation
* go mod vendor
* clean up build.go
* protobuf literals in tests
* downgrade gogo/protobuf
This makes the device ID a real type that can be used in the protobuf
schema. That avoids the juggling back and forth from []byte in a bunch
of places and simplifies the code.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3695