Privacy upgrades to the Zcash light client protocol

TODO:

• First pass at collecting requirements.
• Find the other discussions that were on the forum.
• List all of the features in the current protocol (including ZecWallet's changes).
• Finish reading some research on these kinds of protocols.
• Sketch of a constant-bandwidth container protocol.
• Look at how gRPC is used within the wallets and estimate the effort to switch to a container protocol.
• Sketch how wallets will either download-all or download-some memos.
• Write up informal but complete protocol.
• Collect early feedback from all stakeholders.
• Write detailed ZIP.