tmf: Automatically sync experiments set up with the same hosts

Automatically correct the clock offset of traces inside an experiment
who define a custom property "clock_offset", by calculating
the average of those offsets for all concerned traces.

This is useful for LTTng traces coming from different domains.

This behaviour is similar to how Babeltrace handles traces with
clocks with different offsets.

Bug: 484620

Change-Id: Ic30837fd6e9fc8b9a7592a1722f0240145c77d62
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/65330
Reviewed-by: Hudson CI