Switch RPCs to use strings instead of bytes

[ngcgarcia]

Issue

Our RPC messages currently use the bytes type for ids (128 bit UUIDs). However, bytes can complicate code and make running the code more difficult. It would be easier to both read and run if we used the string type instead for ids.

Next steps

1. Change protobuf
2. Make changes in FinAppService, the SpannerDaoInterface and implementations, and wherever else is needed