Improve fingerprinting of documents

Fixes two issues:
- #4456 : The first 100 bytes are often not unique as they can be
filled with standard PDF headers - so we use the first 200 KB instead.
(This may be overkill)
- Some documents we encountered have invalid xref ids, which were
always coming out as ‘0000000000000000’ - so we detect that and use the
MD5 instead.

Note: This does change the fingerprint for all previous documents, so settings etc would be lost. Maybe they should be migrated or is that acceptable?
Could resetting the stream position break something else?