Scheduled jobs walk again in 4.7

Basically CRM-17669 caused a regression of CRM-16276.

Both CRM-16276 and this StackExchange answer describe the problem. The fix that was incorporated into 4.6.3 was to evaluate the year-month-day of the last run and compare it to the current year-month-day. The result is that daily jobs ran at midnight and hourly jobs ran at the top of the hour.

In adding new scheduled job frequencies, the commit for the new feature switched back to the old logic of checking how much time has elapsed since the last run. That logic causes the walk.