Prepare for transitioning NodeStatisticsAger

NodeStatisticsAger is currently the object which tracks per-node state,
synchronizing our view of what comes from the network vs. what is timed
out. Previous patches have already added partial ordering, this patch
moves the actual modifications in NodeStatisticsAger. This gives us
predictable behavior while keeping us per-switch scalable.

Change-Id: I7a11cf5eb885b46972ecf7a362a8d80a97923a0a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Prepare for transitioning NodeStatisticsAger"