BUG/MEDIUM: resolvers: Add a task on servers to check SRV resolution status

BUG/MEDIUM: resolvers: Add a task on servers to check SRV resolution status

When a server relies on a SRV resolution, a task is created to clean it up
(fqdn/port and address) when the SRV resolution is considered as outdated
(based on the resolvers 'timeout' value). It is only possible if the server
inherits outdated info from a state file and is no longer selected to be
attached to a SRV item. Note that most of time, a server is attached to a
SRV item. Thus when the item becomes obsolete, the server is cleaned
up.

It is important to have such task to be sure the server will be free again
to have a chance to be resolved again with fresh information. Of course,
this patch is a workaround to solve a design issue. But there is no other
obvious way to fix it without rewritting all the resolvers part. And it must
be backportable.

This patch relies on following commits:
 * MINOR: resolvers: Clean server in a dedicated function when removing a SRV item
 * MINOR: resolvers: Remove server from named_servers tree when removing a SRV item

All the series must be backported as far as 2.2 after some observation
period. Backports to 2.0 and 1.8 must be evaluated.