Move stats caching to FM StatisticsManager

This commit is towards HA functionality and includes:
- Cache flow, port, table and description statistics in a cluster
  allocated cache in functional module Statistics Manager
- Add necessary interfaces to allow plugin stats updates
  to propagate up to Stats Manager FM:
  * IOFStatisticsListener - notifies plugin internal components
  * IReadFilterInternalListener - notifies plugin internal
    components (demuxed to containers).
  * IPluginOutReadService - SAL hook for plugins to publish reader
    notifications.
  * IReadServiceListener - SAL service, publishes reader notifications
- Change statistics manager implementation to return cluster
  cached stats instead of calling reader service (Reader service is
  still available).
- Style fixes
- Bug fix: getOFFlowStatistics(switch,match) should consider priority.

Change-Id: I7c7a32102ea43bd4e05444527ad73d2610958603
Signed-off-by: Yevgeny Khodorkovsky <ykhodork@cisco.com>