Add OAuthMiddleware for API

Currently for calls such as ISubscriptionSet.getForOAuthToken() we're doing some sort of authentication ourselves by passing the token and identifier and then using a special private verify_request() call to the identity provider. This worked fine when it was for a request which is not called often, but if we want to enable the client to request/poll a subscription to see its status, either we:

1) verify every request with the identity provider - slow, or
2) create some sort of data store for identities that we've verified, but...

this second option seems unnecessary as this is what open auth is for. We should be able to instead allow the client to make OAuth-authenticated api requests by using the lazr.authentication OAuthMiddleware and adding an OAuth DataStore (from the little I've read so far). This would be more efficient as the identity provider would be contacted only once per token, and would mean that we're relying on standard oauth infrastructure, rather than creating our own less-thought-out solution.