committing the Protobuf implementation for posterity, but will replace
it with json, as Protobuf measures about 6x slower for some workloads
like 'select * from notes'
- Adding files inside Anki now updates the media DB, so a full file
scan at sync time is no longer required if no other changes have been
made.
- Use a protobuf message for backend initialization, and return a
string error if initialization fails.