timing: Add/update some toString methods in callgraph

timing: Add/update some toString methods in callgraph

It makes it easier to debug the callgraph code without having to deeply
inspect each object.

Change-Id: I9c35d570e13f1d4b506df4d83450e83a1c1dfd64
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/88902
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>