Merger annotations for common subject and prune unused annotations

[gklyne]

[LOW PRIORITY]

Currently, creating multiple annotations for a resource results in multiple annotation bodies. I think this is because old annotation bodies are not deleted, but may be because annotations for a resource are not merged into a single annotation stub+body. This isn't really a functional problem, but can be confusing is one looks into the innards of the RO structure.

[gklyne]

Some further examination shows that annotations on a common subject are merged into a new annotation file, but that old (unreferenced) annotation files are not deleted.