Move flow statistics update handling

This finishes the cleanup of StatisticsUpdateCommiter, we now have all
per-node statistics processing guarded by the lock -- e.g. have
predictable and isolated updates to statistics.

Change-Id: Ia58983d5d1dc857e5239a68e3e94fc7d3676cf6d
Signed-off-by: Robert Varga <rovarga@cisco.com>