NFV configuration benchmark

This branch factors out `load_config` from `designs/neutron/snabbnfv_traffic` to `lib.nfv.config.load`/`lib.nfv.config.apply` and adds a benchmark `designs/bench/nfvconfig` which measures their performance.

This branch also fixes a few bugs in related code paths.

See this [wiki page](https://github.com/eugeneia/snabbswitch/wiki/Snabb-NFV-benchmarking).