analysis: basic checks for acyclic property of TmfGraph

analysis: basic checks for acyclic property of TmfGraph

The TmfGraph is Directed Acyclic Graph (DAG). This patch adds three
checks:

 * verify that a node is not linked to itself (this was possible because
the timestamp is greater or equal to itself).
 * assert that getHead() will terminate in the special case where the
tail points to the head indirectly.
 * fix infinite loop bug with TmfGraph.scanLineTraverse() with a
specific graph.

Change-Id: I68a3eaed2c1098df1547d1fba34c35bc1d038404
Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Reviewed-on: https://git.eclipse.org/r/58682
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>