zcash_client_backend: Create the framework for an in-memory wallet backend

[str4d]

We have a MockWalletDb, but that literally just implements the WalletRead etc. traits with dummy data. In order to make the zcash_client_sqlite tests portable to other wallet backends, we should write an in-memory wallet backend that has no persistence, and stores all of the wallet state directly in memory. This is also a good way for the ZF engineers to gain experience with the zcash_client_backend traits.

As a first step, let's put together the framework for this, with todo!()s for all of the trait method implementations.