Implementing a smart version of the rescanExisting flag

Now, the old behavior has been resurrected, and a new one has been
added:

rescanExisting=false, means only add/remove nodes from the DB skipping
the scan phase.

rescanExisting=true (default), perform all the DB updates and execute
the scan phase.

rescanExisting=dbonly, perform all the DB updates skipping the scan
phase.
Improving logging for rescanExisting and fixing JUnit tests.
Merge remote-tracking branch 'origin/jira/NMS-6759-1.14' into 1.14.0