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
yup, those globals should be replaced with a `chan` in `com` since the output pool is shared across all actors. As for size, let me try the suggestion to move utxo as soon as it matures and fix the size to 100.