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).
max_connections is separate from max goroutines, we need both toggles. It will likely be preferable to restrict max push notifications at once rather than max connections.