[ZSA] Ensure that ZIP 209-style issuance tracking is applied to ZSA assets

[daira]

Note that this interacts with #843 : if we do pool value balance tracking for all assets, then this has to be included in the state.

In the case of ZSAs, it is only currently possible for an asset pool value balance underflow to occur as a results of burns. In future there may be multiple shielded protocols, and so there may be a need to transfer value in ZSA assets from one shielded protocol to another, via a turnstile that reveals the transferred balance. Alternatively, other mechanisms to allow non-ZEC value to exit a shielded pool may be added, for example to support bridging.