Move MultipartMessageManager into NodeStatisticsHandler

There is no reason to have a global manager, now each
NodeStatisticsHandler instantiates its own manager -- thus the nodeId
in its tables has no point. This has the added value of being protected
by the NodeStatisticsHandler lock, thus we are prepared to having a
completely consistent view of what operations are really expected even
when switches flap.

Change-Id: I4ca63982506c6f290967040d5626400cb0a0996e
Signed-off-by: Robert Varga <rovarga@cisco.com>
Move statistics request methods into NodeStatisticsHandler

This move make sure we route all requests through a particular node
handler, so it now sees which stats are actually requested for a
particular node.

This means that MultipartMessageManager can use a simple map, as all
operations are now protected by the NodeStatisticsHandler's lock.

Current code organization now allows the individual statistics request
methods to be moved into individual trackers.

Change-Id: Iebd53d6dd447db7ab684cd2c6a90d9abf1b0e91e
Signed-off-by: Robert Varga <rovarga@cisco.com>