Cache incoming viewing keys and detect IVK collisions when adding accounts

[daira]

In #1489 we added a check for collisions between FVK components when adding accounts.

As part of supporting Incoming accounts, we should:

• Use a migration to add orchard_ivk_item_cache, sapling_ivk_item_cache, and p2pkh_ivk_item_cache to the addresses table in addition to the existing orchard_fvk_item_cache, sapling_fvk_item_cache, and p2pkh_fvk_item_cache, and make all of these UNIQUE.
• Change the FVK collision check to an IVK collision check. (If two accounts collide on FVK then they necessarily also collide on IVK, and there is negligible chance that two different FVKs produce colliding IVKs.)

These don't need to block on other changes for IVK support.