Enumerate the data / state that needs to be migrated from wallet.dat to a future full node wallet

[str4d]

Currently zcashd supports exporting wallet keys in a "key dump", but this leaves behind both a bunch of state about how those keys relate to each other, as well as various kinds of state that are only persisted locally and are not recoverable from the chain.

We should go through the zcashd wallet code and wallet.dat file format, and enumerate all the kinds of data that we need to either migrate out of zcashd, or provide an alternative migration pathway for.