Update API to seperate moderation and fetch functionality

[bhaskarkishore]

Update the API based on proposal in ticket #112. The following changes are in scope:

• A new method called moderate is to be introduced. This method allows clients to query the moderation status of a content resource. The method should be non blocking, i.e should return a pending state if the proxy is working on updating the moderation status of the requested content. This method should not return the actual content. The method should suggest a moderation action for the client. The ultimate decision of whether to block or not is left to the client.
• The existing fetch method is to be repurposed to only return cached content. The method should not block in case the requested content is not in the cache and is being fetched.