BUG/MEDIUM: resolvers: Trigger a DNS resolution if an ADD item is obsolete

BUG/MEDIUM: resolvers: Trigger a DNS resolution if an ADD item is obsolete

When a ADD item attached to a SRV item is removed because it is obsolete, we
must trigger a DNS resolution to be sure the hostname still resolves or
not. There is no other way to be the entry is still valid. And we cannot set
the server in RMAINT immediatly, because a DNS server may be inconsitent and
may stop to add some additionnal records.

The opposite is also true. If a valid ADD item is still attached to a SRV
item, any DNS resolution must be stopped. There is no reason to perform
extra resolution in this case.

This patch must be backported as far as 2.2.