ss: accelerate state system creation by 25% by coalescing intervals

ss: accelerate state system creation by 25% by coalescing intervals

The state system threaded back end sends one interval at a time to
a blocking queue to be written to disk. This patch makes it send several
intervals at a time, meaning we have several times less context switches
and several times less blocked threads.

Change-Id: I5cf26da2f1593749245bb6cf5c6f3c5ed65f00e0
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/45603
Reviewed-by: Hudson CI
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>