BUG/MEDIUM: dns: Don't store additional records in a linked-list

BUG/MEDIUM: dns: Don't store additional records in a linked-list

A SRV record keeps a reference on the corresponding additional record, if
any. But this additional record is also inserted in a separate linked-list into
the dns response. The problems arise when obsolete additional records are
released. The additional records list is purged but the SRV records always
reference these objects, leading to an undefined behavior. Worst, this happens
very quickly because additional records are never renewed. Thus, once received,
an additional record will always expire.

Now, the addtional record are only associated to a SRV record or simply
ignored. And the last version is always used.

This patch helps to fix the issue #841. It must be backported to 2.2.