Enable UTXOs spent in expired shielding transactions to be spendable

[str4d]

Currently zcash_client_sqlite assumes that once a UTXO is shielded, it never needs to be spendable again. However, shielding transactions are created with an expiry height the same as any other transaction, so we need to handle this in some way.

The trickiest part is likely to be how we handle two competing chain tips where a shielding transaction is mined on one chain, and expired on the other.

Opened from https://github.com/zcash/librustzcash/pull/985#discussion_r1333774156.