Conflicit Modification Exception on napt-Switch

Conflicit Modification Exception on napt-Switch

Description :
During External-network suite clean-up, its been observed that both
the listener RouterDpnChangeListner.remove() and
ExternalRouterListener.remove() triggered symultanously both of
which updating router-to-napt-Switch DS resulting in conflicting
modifiction exception.

Changes are done for RouterDpnChangeListner to use DJC with router
lock to prevent such conflicting Modification exception.

Issue: NETVIRT-1040

Change-Id: I35ec9e2eaffb537e1f99361cdccda70ec4214d4d
Signed-off-by: xcheara <chetan.arakere@altencalsoftlabs.com>