[NF] Adding IPv4 utility functions class
[IM] Small improvements to ipv4ToARPA()

- Use a more specific network exception
- Not the job of this function to actually strictly validate an IPv4 address - that should happen somewhere else
- We can do a lose validation based on whether we have four octets or not
- The constitutent parts are integers rather than strings - the advantage is any leading zeros will be dropped
[IM] Check if a cookie exists before trying to use it
Merge branch 'nf-net'