'Fetch updates from the peer repository and import them
into the local database (active pull)
@param task: the synchronization task (sync_task Row)
@param onconflict: callback for automatic conflict resolution
@return: tuple (error, mtime), with error=None if successful,
else error=message, and mtime=modification timestamp
of the youngest record sent'

def pull(self, task, onconflict=None):

    raise NotImplementedError
