Account for existing input amounts throughout coin selection

Account for existing input amounts throughout coin selection

We'd previously ignore the existing amount transactions were already
attempting to spend when deciding whether we should add more inputs
throughout coin selection. This would result in us attaching more inputs
than necessary to satisfy our target amount. In the case of HTLC
transactions, we'd burn the HTLC amount completely, since the pre-signed
transaction has zero fee (input amount == output amount).

Along the way, we also fix the slight overpayment in anchor
transactions. We now properly account for the fees the transaction
already paid for, simply by pretending the fees are part of the anchor
input amount.