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).
Better yet, I'll add `Status()` methods to `Locator` and `PropPinger` so that we can include them in the report.