MEDIUM: server: close idle conn on server deletion

MEDIUM: server: close idle conn on server deletion

To be able to delete a server, a number of preconditions must be
validated to ensure it is not in used anymore. Previously, if idle
connections were stored in the server, the deletion was cancelled. No
action was implemented to force idle connection closure, the only
solution was to wait for the periodic purging to be achieved.

This is an extra burden to be able to delete a server. Indeed, idle
connections are by definition inactive and can be closed prior to delete
a server. This is the exact purpose of this patch.

Idle connections removal is implemented inside "delete server" handler,
once it has been determined that the server can be freely removed. A
simple loop is run to call conn_release() over each idle connections.
Takeover is also executed before conn_release() to ensure tasks/tasklets
or any other sensible elements are not deleted from a foreign thread.

This patch should reduce the occurence of rejected "delete server"
execution, especially when connection reuse is high.