analysis: use a null backend for the kernel analysis benchmark

analysis: use a null backend for the kernel analysis benchmark

This adds a benchmark that does not save the state system to disk. It
allows to benchmark only the analysis (and the reading of the events).

Change-Id: I271064111a4c55cf3010c66b3c9c6998d2444a27
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/66853
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI