Wallet should really be the container for anything bip32 related, so
I'd like to slowly wean off of `ld->bip32_base` in favor of
`ld->wallet->bip32_base`
We'll re-use them a few times so having them at a central location is
nice. We also fix a bug that was unreserving UTXO entries upon free,
instead of promoting them to being spent.
Since we have a simple way to query the database for UTXOs we can
simplify some of the coin selection logic. That gets rid of the
in-memory list of UTXOs.
Christian Decker