Memory hoarding in TTree

A large memory burst is observed when filling a TTree, consisting of branches with TClonesArrays as elements.

The main observation is that the filling process leads to a 4-fold total memory consumption.
For example, we observed that when having roughly 800MB of relevant data in memory, ROOT IO causes this to jump to over 2.4GB at the moment of TTree::Fill().

The attached massif output documents the problem.