Add an optional per-host rate limit. This currently only applies to the top-
level navigation. This makes it simpler to implement and simpler to reason
about. The full load of a page is only ever delayed at its head, not
sporadically through the page loading.
The use-case where a RateLimiter is most useful is when the browser is crawling
multiple pages of the same site, and in that case, the top-level rate limit
still applies some degree of limit to any linked resources (e.g. a JS on a
different host).
`--http-nav-delay` is the delay, in milliseconds, to apply to top level
navigates per host. Currently defaults to 0 (disabled).
`--http-nav-burst` is the burst allowed per host. Defaults to 1 (has no impact
when `--http-nav-delay` is disabled).
