tx curve - update utxo selection

if the tx/block in the tx curve is very high, we might run out of utxos to use for creating tx; need to make sure we have enough utxos before generating the curve.

*edit*

more debugging showed that the cause of the following error wasn't lack of utxos but a flaw in the utxo selection process, so updated this issue accordingly
If you need to save a bunch of these in memory while blocks are being attached before the simulation actually starts, I'd use a better type.  Replace the TxId string and Vout uint32 with btcwire.OutPoint.  Replace the Amount float64 with a btcutil.Amount.  I'd remove the Spent bool from this structure as well, as it is not used and once spent, you don't want to keep these around in memory anyways.