Rename NodeStatisticsAger and StatisticsUpdateCommiter

The component's responsibilities have changed, rename them to reflect
this change. The new division of responsibilities is:

There is a single StatisticsListener listens for notifications
carrying the statistics data and routes them towards the appropriate
NodeStatisticsHandler. Once MD-SAL is able to route notifications based
on key, this class should be completely removed.

There are per-node NodeStatisticsHandlers, each of which is responsible
for taking care of lifecycle of statistics for a particular node.

Change-Id: I16d7a1f353a1f484db11da1e5edc97b4778dad3c
Signed-off-by: Robert Varga <rovarga@cisco.com>