OwnIdentity.setEdition(): Use mInsertURIString

Also improve code quality:
- Document why we don't have to delete mCurrentEditionFetchState from
the database
- Add an assert() before the if(edition > ...) to clarify why we don't
handle the case of if(edition < ...)
OwnIdentity.forceSetEdition(): Use mInsertURIString
OwnIdentity.markForRefetch(): Log error if called

This function probably does not make sense for OwnIdentity, only for the
parent class Identity. Thus log an error.
Do not throw because I don't have the time right now for checking
whether it is really never called.