Backport nexthop group synchronisation fixes to 8.5.

[sysvinit]

We're running 8.5.4 in our EVPN-VXLAN lab setup, and I've run into the issues with nexthop group synchronisation which were fixed in 9.x in #13413 and #14080. It would be great if these fixes can be backported to the 8.5 branch, as we're currently carrying these fixes as patches in our local build.

[donaldsharp]

If you have backported these two PR's already, feel free to add a backport PR. I see no reason to not bring these back to that branch

[ton31337]

#15364, #15363, #15362