VPP debug image with worker threads hit assert on adding IP route with traffic (VPP...

VPP debug image with worker threads hit assert on adding IP route with traffic (VPP-892)

When stacking DPOs the VLIB graph is also updated to add the edge between the nodes, if this edge does not yet exist. This addition should be done with the workers stopped.

Change-Id: I327e4d7d26f0b23eb280f17e4619ff2093ff7940
Signed-off-by: Neale Ranns <nranns@cisco.com>
(cherry picked from commit c02bd03ddf5eec9e9c79811360685f13e4ba8ee1)