Add rate limiting

This pull request adds support for rejecting incoming connections if too many goroutines are active at once, and rate limits HTTP requests from origin IPs. `RateLimitedHandler` uses a token bucket implementation ported from [`node-restify`](https://github.com/mcavage/node-restify).
Do you think it's worth tracking what could have caused SplitHostPort to fail before we handle the request?

(I'd probably note it as a Warning at least.)