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>