reset milliseconds on DateTime return values

[larsjaas]

cron4s currently preserves the milliseconds of the from value to the next() function and keeps it in the return value.

Let's say a task takes 100ms to perform. I set up "* * * * * *" as the CronExpr. Each time after the task is performed, I ask CronExpr to find the next once-a-second time based on the current time. I would expect to perform the task 60 times a minute, only slipping if the task itself lasted longer than one second. But because of drift caused by the current implementation I guess it would only be performed 54 times.

To avoid this kind of drift in cron-expressions that have periods of something triggering every second, the milliseconds-part of the returned time must be set to 0 (or you would have to keep an "origin" time around to use as milliseconds reference, which seems like a lot of extra complexity for no gain).