Consider inverting juice

[mikera]

Currently juice gets reserved (taken from balance) at the start of the transaction and counts down towards zero.

This has a couple of issues:

• Reservation of juice affects balance within a transaction, which is a big confusing
• It would be easier to interpret juice as "juice used" rather than "juice remaining" in many cases.

Consider inverting the numbers so it counts up from 0 to some pre-set limit? Might need to check if limit is still valid whenever balance changes