lttng.ust: Consider all statedump events to happen at SD start

Use the statedump:start event's timestamp as the start time of
library load mappings, instead of the individual bin_info
or related build_id/debug_link events.

This will allow trace events happening concurrently with a
statedump to display their debug-information correctly.

Bug: 497943

Change-Id: I0d3474ad7e45f81b370e368664a234b4c5c38499
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/77375
Reviewed-by: Hudson CI