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
We need to hold the coinbase outputs of all mature tx so atleast `16200` size for `utxo` is definitely required. `stxo` could probably be unbuffered actually, let me check that.