Better HD wallet scanning.

[schancel]

Currently, we re-used change addresses. For privacy reasons we should not be doing that. However, given the number of transactions we send we need to fix the wallet to scan HD wallets using a binary search for active addresses. They should only count the first UTXO on any address, as we want to explicitly deny mixing random dusting transactions from being included in any future payments. These amounts should be cordoned and coinshuffled into new addresses to avoid linkage.