MASP finalization plan

[cwgoes]

Proposed order after reading through the issue backlog:

1. Investigate the epoch transition proof issue and agree on a plan
2. Convert MASP to a native validity predicate (rebase and merge masp: move to native VP #2051)
3. Check that our imported versions of the upstream MASP crates are the latest
4. Implementing missing validation (Missing MASP Transaction Validation #1373 and Masp/Base boundary conditions #1962)
5. Check/fix unsafe validation (Unsafe MASP Transaction Validation #1374)
6. Audit MASP & MASP rewards
7. Other stuff

[adrianbrink]

Ensure that wallets have an easy library to import to generate proofs that we can independently optimize.

[adrianbrink]

also check that proofing times don't scale with epoch sizes.

[cwgoes]

Two points to check on the epoch transition issue:

1. We should allow proofs with reference to past commitment tree anchors.
2. We should allow unshielding from previous epochs (the user will receive fewer rewards, we can show a warning in extreme cases).

Together per discussion we think that these two changes should resolve the issue.

[cwgoes]

Notes:

• Rework to avoid unnecessary conversions on AssetType, which should be used all the way through to the edge of CLI client input.
• Discrete conversions with threshold: amounts below conversion are currently left behind - we should just send them anyways, and later allow unshielding (with no conversions)
• Check on proof generation time increasing (maybe more notes to convert?)
• Check on duplicate SpendDescriptions, OutputDescriptions, etc. - seems to be the case in recent iterations of the client.

[cwgoes]

• Fix MASP verifying key / parameters loading (should only load once into memory when the ledger starts) (see reference here)