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
```Go
for {
    a := <-c
    ...
}
```

and the single case select statement you have can be replaced with the more idiomatic

```Go
for a := range c {
    ...
}
```