vlib: introduce lazy next node initialization

vlib: introduce lazy next node initialization

This change allow a node registration A to name a next node B that does not
exist yet at registration time. When node B is effectively created,
vlib_node_main_lazy_next_update need to be called so that the vlib graph
is updated accordingly. To enable this behavior, node A needs to bear
the new VLIB_NODE_FLAG_ALLOW_LAZY_NEXT_NODES.

Change-Id: I561d3a0de19a0b7bd1045760a2ba8e27d27caa9a
Type: improvement
Signed-off-by: Mohammed Hawari <[email protected]>