Bug 740: Forwardingrules-Manager support for DataChangeListeners

+ Bug 619 - Eliminate the use of xtend in md-sal/forwardingrules-manager

migration from xtend to java files FRM
add DataChangeListener to FRM (Flow, Meter, Group)
remove FRM handlers (Flow, Meter, Group)
fix delete changes registration (in ImmutableDataChangeEvent -> old DataStore)

Change-Id: I56003833519d9689a489ed5f77b3f7fc4ac2e204
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>
Merge "Bug 740: Forwardingrules-Manager support for DataChangeListeners"