'Returns whether user_id has elected to discard history for room_id.
Returns False if they have since re-joined.'

@cachedInlineCallbacks(num_args=2)
def did_forget(self, user_id, room_id):

    def f(txn):
        sql = 'SELECT        COUNT(*)    FROM        room_memberships    WHERE        user_id    =    ?    AND        room_id    =    ?    AND        forgotten    =    0'
        txn.execute(sql, (user_id, room_id))
        rows = txn.fetchall()
        return rows[0][0]
    count = (yield self.runInteraction('did_forget_membership', f))
    defer.returnValue((count == 0))
