reduce memory consumption of QP zone and cache databases

every node of a QP database contains a copy of the nodename, which is used as the key for the QP-trie. previously, the name was stored as a dns_fixedname object, which has room for up to 255 characters. we can reduce the space consumed by dynamically allocating a dns_name object that's just long enough for the name to be stored.

Closes #4614 (closed)