[caldav] Save etags returned from PUT requests, and save them to the database instead of as custom incidence properties

removeEntries() deletes all entries for the notebook from the db, but now that the db also holds etags, the etags shouldn't be cleared on every session like other entries, so I added removeIncidenceChangeEntriesOnly() which only deletes non-etag entries (i.e. additions, modifications and deletions).