feature: store arc index in vlib_buffer

feature: store arc index in vlib_buffer

Previos implementation imposed a limit that node can be member of
only one feature arc as feature arc index was stored in node runtime.
Now start node stores index into vlib_buffer_t and feature
nodes are taking it from there.

Change-Id: I252841083bc292af7ed36bf1ac652f2b8c16bfb8
Signed-off-by: Damjan Marion <damarion@cisco.com>