BUG 6841: Few Remote flows not deleted on DPNs

BUG 6841: Few Remote flows not deleted on DPNs

Few Remote flows not deleted on DPNs after migration from
internal to external VPN.
For a vpn associate/dissociate, the vpn interface update event wait was
getting accumulated for every interface across vpns. So, vpn manger was
getting removed before interfaces were cleaned.
A label of 0 was being used to create unique ids across vpn engine if
IdManager failed to return a valid label.
Java Synchronize has been replaced by LockManager to handle events
gracefully across PLs during simultaneous northbound and southbound
events for subnet routes.

Change-Id: I3cf1d1553bd6b6b025a9cedbc860b7af246f1afe
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>