timing.core: add merge function to segment store

timing.core: add merge function to segment store

This allows merging statistics node. This will make statistics
work in a more streamlined way.

Note: merging introduces a slight error to standard deviation,
This is due to the pooled variance algorithm used.

Potential use-cases for this:
* Map-reduce statistics on a segment store.
* Merging trees of statistics.

Change-Id: Ie6758bdcd5df03b58dc5521bf07fa5f9693c30bf
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/79763
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI