Split scanning into 'quick scan' and 'full scan'

[cygnet3]

Right now when we initiate a scan, we are scanning both the inputs and the outputs. The inputs are scanned to find owned outputs that are being spent.

Scanning the outputs is pretty much always needed. Scanning the inputs however, is only needed on these occasions:

1. After a sending transaction, check if the tx has been confirmed
2. If you have another wallet that uses the same seed, and you spend an owned input using another wallet.
3. If you reset the scan height, and deleted some of your transaction history

Situation (1) is very predictable, since we initiate this ourselves. To avoid (2) we should tell users not to recover their wallet on 2 separate devices. (3) is related to resync issues. We probably need to investigate this separately, but this should only occur very rarely.

This means, that the vast majority of the time users are only interested in scanning the outputs. To accommodate for this, I think we should have a 'quick scan' feature that only scans the outputs. This can have significant performance improvements, but has a complexity tradeoff. But I think this tradeoff is worth it.