Send BGP flag to retain same nat port across flows.

In BGP as service same nat port is used for different CN peers.
Now if one CN is going down agent will send delete for flow, which in turn will
reset the port still in use by second CN. Now because of this reset packets from
second CN will start going to vhost. This will cause session reset for second CN
and in turn other issues arise.

Solution:
New flag tells vrouter to retain the port for BGP flows even if flow is deleted.

Change-Id: Ib4796f631e3ded3ed814da386cdf182d1c2c9e6b
Closes-bug: #1551576

src/vnsw/agent/vrouter/ksync/flowtable_ksync.cc

@@ -325,6 +325,9 @@ int FlowTableKSyncEntry::Encode(sandesh_op::type op, char *buf, int buf_len) {
             if (nat_flow->is_flags_set(FlowEntry::LinkLocalBindLocalSrcPort) ||
                 nat_flow->is_flags_set(FlowEntry::BgpRouterService)) {
                 flags |= VR_FLOW_FLAG_LINK_LOCAL;
+                if (nat_flow->is_flags_set(FlowEntry::BgpRouterService)) {
+                    flags |= VR_FLOW_BGP_SERVICE;
+                }
             }
 
             flags |= VR_FLOW_FLAG_VRFT;