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).
We're duplicating the same status for both Healthy (over all health report) and StoreHealthy (storage health report)? Can we drop StoreHealthy if we're not distinguishing it?