Move MultipartMessageManager into NodeStatisticsHandler

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>