Function append

src/bthread/key.cpp:251–263

    }

    void append(KeyTable* keytable) {
        if (keytable == NULL) {
            return;
        }
        if (_head == NULL) {
            _head = _tail = keytable;
        } else {
            _tail->next = keytable;
            _tail = keytable;
        }
        keytable->next = NULL;
        _length++;
    }

    KeyTable* remove_front() {
        if (_head == NULL) {