Avoid potential for linking unified addresses when shielding transparent funds.

[nuttycom]

#56 is a stopgap measure for auto-shielding that will currently link all diversified unified addresses that have received transparent funds since the last shielding operation, by way of creating a transaction that shields funds from the transparent receivers of each address. This is only safe so long as wallets don't start using the functionality to roll addresses, and so should be fixed as soon as possible, but the fix is not straightforward because even shielding in separate transactions could effectively link the addresses via guesses about the timing of the shielding operations.