Guardian: consider making an optional call to claim() every time

[Ivshti]

Current design: when a channel is closed, we allow users to regain their unspent amount plus the late payment interest, but we only call claim() when the contract runs out of funds to pay. This means that the last callers will rely entirely on claim() to get their funds.

Consider this design: in every getRefund(), transfer the unspent amount (totalDeposited - spenderLeafAmount), and optionally pay out the additional late payment interest through calling claim()

The drawback is gas usage, so it must be measured.

This way, we guarantee that the unspent amount will always be paid, and if there's some issue with claim(), only the interest will be omitted

Some gas can be saved from not keeping track of remaining

There is a third option: call claim() only when the channel is closed and from that point onward, distribute *multiplier. The consideration there is that we need to know whether claim() was called (whether interest exists) on subsequent calls, and we need to ensure that we use the correct interest rate (whatever it was at time of calling), considering it's mutable.

[Ivshti]

Decided to do it after all, merged into #identity-tests