Consolidate document equivalence support

[csillag]

As a user, I would expect cross-format annotations to work reliably; if several documents are linked together by metadata, annotations made on any of them should show up on all of them.

However, today that's not always the case.

Instead of proper requirements, I will just link the main issue:

• [x] https://github.com/openannotation/annotator-store/issues/110 - Multiple document equivalences