only rate limit requests that return data

[lanrat]

Requests to pages that do not return data should not be rate-limited.

For example: js, css, static assets should not be rate limited.

Adding a flag/option to the router that can opt a page out of rate-limiting is probably the best way to go. However the default should be for rate-limiting to be enabled.

ex: https://github.com/CAIDA/dzdb-web/blob/master/app/web.go#L52