ctf: Fix race condition in ctfiterator

ctf: Fix race condition in ctfiterator

You could advance an event while seeking, that is no longer possible
performance droped from 78000 events/s reading to 75000 events/s.
Potential hazard, threads can access the iterator and advance and seek
concurently. This is allowed by design, it's an iterator, it should be
wrapped in a safer candy coated shell so it doesn't melt in you hands.

Change-Id: I4df50389728f075f15312107f44847d61343366e
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/14060
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>