Create a new token bucket rate limiter that limits globally at 'g' requests/sec and per-host at 'p' requests/sec; It remembers the rate of the 'cachesize' most recent hosts (and their limits).

Domains we never even make HTTP requests too.

var ErrBadHead = fmt.Errorf("url failed head check").

Limiter controls how frequently events are allowed to happen globally or per-host.