BUG/MEDIUM: resolvers: Make resolution owns its hostname_dn value

BUG/MEDIUM: resolvers: Make resolution owns its hostname_dn value

The commit 37abe56b1 ("BUG/MEDIUM: resolvers: Properly cache do-resolv
resolution") introduced a regression. A resolution does not own its
hostname_dn value, it is a pointer on the first request value. But since the
commit above, it is possible to have orphan resolution, with no
requester. So it is important to modify the resolutions to make it owns its
hostname_dn value by duplicating it when it is created.

This patch must be backported with the commit above.