Make TransferAllowance an interface

[mattwigway]

The base TransferAllowance class isn't really useful in any real-world system where there are differing transfer allowances per route/route type, because it only counts the number of transfer remaining, not what those transfers are to. I propose making it an interface that just has a boolean method atLeastAsGoodForAllFutureRedemptions, and let every individual transfer allowance handle its state internally. This would avoid confusion when new fare calculators are implemented; I know of at least two cases where this confusion has caused an issue - in the Boston calculator, where we were incorrectly cloning TransferAllowances and losing all Boston-specific information, and in the r5r fare calculator where the targets of the transfers were not being correctly tracked.