Annotations are lost when PDF fingerprint changes

[_I fully recognize that this title of this issue is actually describing the way Hypothesis is meant to work with PDFs, but there are circumstances where this behavior is surprising and/or problematic to users. This issue comes out of a discussion between myeslf, @mkdir-washington-edu , and @robertknight , and is an attempt to describe these concerns._]

Occasionally a user will need to edit or alter a PDF in a way that changes the fingerprint:
- User discovers garbled or unusable text layer and needs to run PDF through OCR again
- Original PDF is lost or deleted from host, and new PDF needs to be found and/or re-uploaded
- Original PDF does not display properly in certain platforms/environments, necessitating creation of a new PDF

All of these scenarios result in the new PDF containing a different fingerprint than the original, thus losing any annotations previous associated with that document.

In these cases, if there was a method to maintain document equivalency, or create an association between the previous version and the new, previous annotations could persist.