Use replay groups for feature overlays

Thanks @elemoine for the majority of the work on this.

fixes https://github.com/openlayers/ol3/issues/1876
It would be nice to have something like `ol.renderer.vector.getSquaredTolerance(resolution, pixelRatio)` so this bit of code with the magic `4` didn't have to be duplicated in 3 places.  And the `getSquaredTolerance` function could use `ol.SIMPLIFY_TOLERANCE` or something (a define that was set to `0.5` device pixels by default).

This could be handled separately.