Make a list of RPC calls we want to implement

[gtank]

Right now, we only have a few that were inherited from parity-bitcoin. As far as I can tell, none of the z_ calls from the Zcash Payment API are implemented or exposed. It's not yet clear to me that we can or want to be exactly RPC-compatible with zcashd but we will at least need to provide similar functionality, so a list is helpful no matter what.