Prevent packet pool starvation when waiting for nh resolution

[david-marchand]

Packets requiring some arp/ndp resolution for nexthop are currently stored in a per interface (fixed size) "hold queue".
In theory, doing so exposes grout to packet starvation if the packet pool size is < sum of all "hold queue" sizes since it is then possible to store more packets in the "hold queues" and then no packet would be left in the packet pool.

[david-marchand]

One solution would be to have a "control path" packet pool.
Held packets would come from this pool, and would be copied from the original mbuf, returned to the "data path" packet pool.