automatically inherit VRF from pool in nipapd

I hope I didn't break anything, please to check ;)

When allocating from a pool, nipapd now looks at the implied VRF of a
pool and sets the same VRF to the new prefix being allocated. It doesn't
work for from-prefix operations as the input argument to from-prefix is
a list of prefixes and not a list of IDs of prefixes, so they can only
be uniquely identified by being specified in combination with a VRF.

Part of #286