Prior to this change, the `mark_transparent_utxo_spent` method assumed
that the UTXO information for outputs belonging to the wallet would be
known to exist before their spends could be detected. However, this is
not true in transparent history recovery: the spends are detected first.
We now cache the information about those spends so that we can then
correctly record the spend when the output being spent is eventually
detected.

At present, data from this spend cache is never deleted. This is because
such deletions could undermine history recovery in some narrow cases
related to chain reorgs.

…ecrypted_tx`

…Request`

…output`

…actions in `store_decrypted_tx`

This permits this method to be used in a migration.

…ore additional transparent history.

…quests do not exceed the chain tip.

`lightwalletd` will return an error in the case that the requested end
height exceeds the chain tip. This should be considered a bug in
lightwalletd, but for now we will work around it in the wallet.