Centralize targetNodeRef in the NodeStatisticsHandler

There's no reason to allocate it over and over again, we will be using
for a long time. Also push the DataProviderService into
NodeStatisticsHandler such that we do not have circular references.

Change-Id: I1e2444644a27516c87ca93d386f5dc2ac334ffaf
Signed-off-by: Robert Varga <rovarga@cisco.com>
Start rework of MultipartMessageManager

- TxIdEntry should be private static final
- Remove unused method
- Use System.nanoTime() for timekeeping

Change-Id: I51bf99082aadeb1f83055a0bf6a07e320c089db0
Signed-off-by: Robert Varga <rovarga@cisco.com>