Add new managed resources implementation

A new ManagedStorage interface allows blobs to be stored in a mongo database. The blobs are initially namespaced to an environment but apis will be added to the interface to allow data to be stored globally or against a given user.

Data is de-duped when Put - if there is already a blob with the same checksums (md5/sha256) it is not stored twice; a reference to the original data is used.
So I was mistaken before, and there's no test for this. It'd be good to see one, but I understand it might be difficult to do.