TMF: Correct bug when synchronizing more than 2 traces

TMF: Correct bug when synchronizing more than 2 traces

Previously, when synchronizing more than 2 traces, it would return false
results when not all traces had a path to all others. Some timestamp transforms
need composition to be accurate wrt the reference trace

Change-Id: Ie71c4063970af5db1b1476d72351639342a149d8
Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/31776
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Hudson CI