Performance suggestion for large marker set, when adding to incrementally

[ikb42]

Hi I'd like to suggest an additional control createOnly be added to the controls returned by getDirectiveControls. This would not attempt to check existing markers as create currently does.

When I measured the create process it was _clean(models, oldModels); and skips = _getNewModelsToSkipp(models, oldModels, leafletMarkers); that were taking most of the time in the execution cycle.

    leafletData.getDirectiveControls()
    .then(function (controls) {
        controlls.markers.createOnly(newMarkers);
    });