If an object exists locally but not remotely and the local version has a
version number, that's an error. I don't think that should ever happen,
but it can if things somehow get out of sync due to other bugs.
To address, reprocess the API delete log during a full sync and then
reset the version number of all remaining local objects that don't exist
remotely (not just unmodified objects, as was the case previously) to 0
for uploading.
When remote deletions are reprocessed, delete local objects that haven't
been modified and show the conflict resolution window for any local
items that have.
Also:
- Clean up checking of last remote library version during download
syncs
- Add Zotero.DataObjects.getAllKeys()