What is the difference between ip_udp_fast_path and ip_udp_slow_path?
It's fast if the incoming packet gets a hit in the packet classifier (e.g. we have
bound endpoint which matches port, address) and we can immediately find the correct
conn_t to handle the packet.
It's slow if we have to look harder for the conn_t to deliver the packet.
This may seem a little bit vague but this is implementation specific with conditions
that are subject to change.