tmf: Code cleanup in the history tree

tmf: Code cleanup in the history tree

The method preOrderPrint wasn't thread safe because it was using
a "global" field. This is not a big issue, but it's cleaner this
way.

I did a cleanup pass on this file.

Change-Id: I6255be52d3b2a8f84ef95a093578e10489c234eb
Signed-off-by: Etienne Bergeron <etienne.bergeron@gmail.com>
Reviewed-on: https://git.eclipse.org/r/19560
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>