Updated book progress model in anticipation of the KOReader sync
feature.
Renamed properties:
- renamed "progress" to "percentage"
- renamed "pdf_loc" to "pdf_page"
- renamed "epub_loc" to "epub_cfi"
New properties:
- "epub_xpointer", marks the location of an epub in KOReader
- "device", the device type that updated the progress
- "device_id", the id of the device that updated the progress
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