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
@tuxcanfly if ```utxo``` is used for coinbase utxo then a good way to use it would be to hold there all the immature coinbase and when every utxo matures, send it into txMap. So ```utxo``` size would be 100.