optimising fork reference interpretation

[nugaon]

By default, the serialized fork reference is pointing to another Mantaray node.
Nevertheless, in plenty of cases, the leaf nodes only have address to a content saved on Swarm and its corresponding metadata can be serialized under forkMetadata.
mantaray-js could stick to a metadata format in forks that change the default fork reference interpretation in order to save unnecessary Mantaray node creations.

The applied metadata format has to be approved in Fair Data Improvement Proposals