Add a configurable idle timeout to stunnel

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>

Needs to go in only after this pull request has gone in : https://github.com/xapi-project/xen-api-libs-transitional/pull/7

Signed-off-by: Akshay <akshay.ramani@citrix.com>