Support for multiple accounts

[AArnott]

It seems we need a high-level issue tracking overall support for multiple accounts:

Here are some work items that occur to me:

• WalletWrite should expose a function that allows adding HD-derived accounts at a particular index. The existing create_account function assumes "last index + 1" which is a great policy, but doesn't provide the app with even an awareness of which index was created. I feel the app should also be able to control this, but with fn-level docs urging the app to only create one account index at a time and in-order.
  • zcash_client_backend: Add add_new_account to the data access API. #628
• The ability to import accounts via UFVK and UIVK. A parameter may indicate whether the caller expects that a spending key is available for this account so that sync knows whether to maintain commitment trees (or whatever the marginal cost is for spendable accounts).
• Adding a second account should trigger partial re-scan. That is, the scan_queue should indicate that blocks must be re-scanned. Ideally, the rescan would only run trial decryptions for recently added accounts that have not yet been scanned.
• Removing accounts
• Any stabilizing changes for intra-wallet, inter-account payments. e.g.
  • zcash_client_sqlite reports transaction orchard output twice, and doubles account_balance_delta #1324
  • zcash_client_{backend,sqlite}: Correctly track accounts that fund each transaction output. #1305
• Other bugs
  • WalletDb::create_account fails adding second account: Attempted to write subtree roots with indices 11..12 which is discontinuous with existing subtree range 16..17. #1389
• Other features
  • Adding a second account with an older birthday height effectively changes height for existing account #1436