Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner_v1.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.

def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner_v1.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.
        """
        self._database = database

        while not self._sessions.full():
            session = self._new_session()
            session.create()
            self._sessions.put(session)