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
From what i understand, the max size of strings that ```stxo``` will ever have to accommodate is the max size of utxos a block can have. Since in btcsim, one utxo = one transaction, then this channel does not need to be larger than the max size of txns a block can have (around 5k in 1MB blocks).

(Probably the same goes for ```utxo``` too.)