Split off tracking of node presence

Introducing FlowCapableTracker. It's sole responsibility is to react
to FlowCapableNodes appearing and disappearing in the inventory and
spinning up statistics handlers based on that. This makes the
NodeStatisticsHandler lifecycle more predictable, thus we can activate
the 'ignore' part of the getStatisticsHandler(). Final part limits the
visibility of the various 'getFooStats' methods, so we can reason about
them better.

Change-Id: Icdbcca6038ae7fb0bff06d378fafa0f476f0fe18
Signed-off-by: Robert Varga <rovarga@cisco.com>