feature: rate limit

[SkohTV]

Fix https://github.com/facebook/bpfilter/issues/215

I've started getting into adding this features. So far, I've managed to add the ratelimit [int]/[time_unit] keyword to the cli and put it's value into a bf_map. ~~However I'm not sure about implementing a "real" rate limiting as it seems to require EMIT (and that look scary).~~ Added (empty for now) elfstub to handle rate limiting

~~The idea of the implementation so far is pretty naive (only allow the first X requests to pass through), mostly because it was easier to implement and I hadn't strong feelings on which direction to go.~~

I'll keep this as a draft for now since it's still in its early stages.