Integrates the HTTP PollingClient with the op-node. To do this, I needed to change the RPC interface to return an `ethereum.Subscription` object rather than a concrete *rpc.ClientSubscription instance. I also introduced a new method to return the correct `client.RPC` implementation based on the user's chosen RPC URL.

Lastly, I added an additional CircleCI job to run all end-to-end tests against an HTTP-based RPC endpoint to prevent issues from arising in the future.