Implement Sideways Data Loading

Problem

This is a first-class API for sideways data loading of stateless (although potentially memoized) data from a global store/network/resource, potentially using props/state as input. [code block] observe() executes after componentWillMount/componentWillUpdate but before render. For each key/value in the record. Subscribe to the Observable in the value. [code block] We allow onNext to be synchronously invoked from subscribe. If it is, we set: [code block] Otherwise we leave it as undefined for the initial render. (Maybe we set it to null?) Then render proceeds as usual. Every time onNext gets invoked, we schedule a new "this.data[key]" which effectively triggers a forcedUpdate on this component. If this is the only change, then observe is not reexecuted (componentWillUpdate -> render -> componentDidUpdate). If props / state changed (i.e. an update from recieveProps or setState), then observe() is reexecuted (during reconciliation). At this point we loop over the new record, and subscribe to all the new Observables. After that, unsubscribe to the previous Observables. [code block] This ordering is important since it allows the provider of data to do reference counting of their cache. I.e. I can cache data for as long as nobody listens to it. If I unsubscribed immediately, then the reference count would go down to zero before I subscribe to the same data again. When a component is unmounted, we automatically unsubscribe from all the active subscriptions. If the new