Refactor CronTrigger.java for readability

[mswatosh]

CronTrigger.java has a number of simply named variables that have overlapping meaning. We should refactor this class for readability.

For example, l is never described, and we see both min and m in some functions (where m is the index of a month)

Comments from my code review:

int d = Arrays.binarySearch(daysOfMonth, dayOfMonth);
int l = Arrays.binarySearch(daysOfMonth, dayOfMonth - lastDayOfMonth - 1);

I did some digging, and from what I see, d isn't (exactly) the day, it's the index of the day in the daysOfMonth array. And l is the offset from the last day of the month in the daysOfMonth array.

int m = Arrays.binarySearch(months, time.getMonthValue());

like above, m is the index of the month in the months array. None of these need to be updated now, but we may want to open an issue to refactor some of this for maintainability