Refactor TimeRange classes

The non-absolute time range objects used for searches currently reevaluate their start and end dates on each method call.
This can lead to strange behavior (as in #2382) where even the same instance of a 60 second relative range returns a different interval when asked repeatedly.

In some other cases (#2335) we need this behavior because the instances stay in memory for a long time, but we want the searches to reflect the time range anchored at `now`.

One option would be to refactor the time range creators to default to `AbsoluteRange` creating instances that never change their interval.
If needed code can also create "reevaluating" instances using different methods which stand out a little more.

``` java
// stays fixed
AbsoluteRange a = RelativeRange.create(60);
// re-evaluates every time
RelativeRange r = RelativeRange.createMoving(60);
```

related to #2382 #2335 #2301