Use bulk-insertion for addFeatures

Right now, method `addFeatures` of `ol.source.Vector` adds feature-by-feature to the R-Tree. With #2867 we can use the method [`load()` ](https://github.com/mourner/rbush#bulk-inserting-data) of rbush, so that all features are inserted at once. Which is faster and yields a better tree.

Closes #1521