Specify light wallet protocol changes as an update to ZIP 307

[daira]

These are the changes to add an RPC method to obtain subtree roots, and to add any necessary information to the output of other RPC calls.

[LarryRuane]

Perhaps we should make a few other minor changes to ZIP 307 to correct for changes made a long time ago, such as using the RPC interface to zcashd, rather than zmq. Also, we've added some client-side gRPCs over the years.

[str4d]

I think this is #1202.