Enable address rotation for Zashi

[nuttycom]

Is your feature request related to a problem? Please describe.

Reusing addresses can be harmful to a user's privacy, because counterparties can compare data to get a more complete view of a user's behavior.

Describe the solution you'd like

Each time the user shares a diversified address with a counterparty, or when a previously-un-seen diversifier index is detected in wallet recovery, that address is marked as used and will not be shown again as a receiving address for the wallet (modulo possibilities related to the "reverse address book" functionality discussed below.)

One possible feature would be to make it possible to request an address for a specific named counterparty. This would enable a sort of "reverse address book" that makes it possible to identify which counterparty an address corresponds to when receiving a transaction. This was suggested here

[daira]

Note that although the description focuses on diversified addresses, the effect will also be to generate distinct Transparent P2PKH Receivers, because UAs generated according to ZIP 316 use the same index within an account for Sapling, Orchard, and P2PKH.

[daira]

As @nuttycom points out, to maintain the greatest possible unlinkability (subject to other parties' behaviour), UTXOs received on different transparent addresses need to be tracked separately and not conmingled when they are shielded. This also makes the assumption that Zashi does not directly support transactions that have both transparent inputs and transparent outputs, i.e. it continues to require shielding of all received transparent funds before they can be further spent. (The implementation of ZIP 320 in Zashi will not contradict this, because it will always send via a distinct ephemeral t-address.)