Drop in-memory copy of schema zip file
    
    The schema cache used a BytesIO buffer to read/write schema cache before
    it got flushed to disk. Since the schema cache is now loaded in one go,
    the temporary buffer is no longer needed.
    
    File locking has been replaced with a temporary file and atomic rename.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: David Kupka <dkupka@redhat.com>