Destroy elements in the root tree when inserting large number of new elements

[felix-halim]

The current C-Tree is inserting elements eagerly. This is bad when the root tree is dense (i.e., it already have many cracks). It no longer "do just enough" as the insert cost will be O(log(N)) instead of O(1).

One way to avoid this cost is to buffer inserts at the root element and push them down as needed.