GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3527
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3792
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