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
Both of those being globals makes me rather wary.

The buffer size is also rather insane.  Another issue is that you're using these as a dumping ground for coinbase transaction outputs before the simulation starts, but the block at which that begins is a command line flag.  What if I wanted to start the simulation at block 20001?