tmf : Add Leaf nodes to the History Tree

tmf : Add Leaf nodes to the History Tree

This patch allows the use of leaf nodes in the History Tree. A leaf node
cannot contain children, so we can save 50 x (int + long) = 600 bytes
for each node (64k).

Note that this patch changes the on-disk file format of the history tree.
FILE_VERSION is bumped accordingly.

Change-Id: I4d99727a23fcdfbdd911fd27c621354864ebfde1
Signed-off-by: Florian Wininger <florian.wininger@polymtl.ca>
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/24791
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>