BUG-582: do not use linear search of child nodes

Profiling has found we are epending some time in addChildNode() to
search all the existing node. As it turns out, we are already using a
TreeSet, so using a TreeMap instead will give us a log(N) lookup
capability at no cost at all.

Change-Id: I5e3962170d8d5ae8bb087de3de52928ad6cebc1c
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-582: do not use linear search of child nodes"