Upstreaming changes in Netvirt

Upstreaming changes in Netvirt

(Dependent on https://git.opendaylight.org/gerrit/#/c/42540/)

This commit includes VPN Engine Concurrency Fixes also some SNAT and DNAT
regression issues.

1. We have made multiple fixes here for safe concurrency.

2. We have moved all helper methods that deal with VRFEntry
resource into FibManager from VPNManager.

3. We have gracefully handled deletion of bgpvpn regardless of
how many ever external routes are available in such vpn.

4. We have fixed NATService to directly use FibManager to
program SNAT/DNAT VRFEntries.

5. As of this review, all local VM routes WILL NOT be batched for
write into VRFEntry Config DataStore.

6. All the external routes imported from BGP, WILL BE batched and
written to VRFEntry Config DataStore.

Change-Id: I3109516e8d163f2b61ba49acc463b85b09bbaf85
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>