Support "Spam Filtering" in Zashi

[nuttycom]

Is your feature request related to a problem? Please describe.

When restoring an older wallet from seed, it takes an unreasonably long time to scan all of the blocks from the period of the "sandblasting" attack. Zashi should expose an option to the user that allows them to skip over transactions with large numbers of shielded outputs in wallet restore.

Describe the solution you'd like

The wallet will scan in two phases: a quick scan that skips the spammy transactions in the initial restore, and then a low-priority task to do a full scan (using background sync) to eventually ensure that no transactions are missed.