@type val: L{bytes}

        @param flags: the flags to store with the key.
        @type flags: L{int}

        @param expireTime: if different from 0, the relative time in seconds
            when the key will be deleted from the store.
        @type expireTime: L{int}

        @return: a deferred that will fire with C{True} if the operation has
            succeeded.
        @rtype: L{Deferred}
        s