Add an event and its corresponding key to the store.

def add_event(self, key, event):
        """Add an event and its corresponding key to the store."""
        if self.key_exists(key):
            # This check might actually also be done further up in the chain
            # (read: SQLiteEventStore). Could potentially be removed if it
            # requires a lot of processor cycles.
            msg = "The key already existed: {0}".format(key)
            raise EventStore.EventKeyAlreadyExistError(msg)

        self._rotate_files_if_needed()

        # Since I guess LogEventStore is less mature codewise than
        # SQLiteEventStore I am writing to that log file first. If something
        # fails we are not writing to SQLiteEventStore.
        for store in self.stores:
            store.add_event(key, event)
        self.count += 1