Add a configurable idle timeout to stunnel

Imported from xen-api.git 53ef46d2491bad21366ffa63cadcc59adfbca34c

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Imported-by: John Else <john.else@citrix.com>

Conflicts:
	ocaml/xapi/xapi.ml
Add configurable idle timeout to stunnel

Imported from xcp-inventory.git 13735def832bff388e6d01c34645a4cef3fc73eb

Signed-off-by: Akshay <akshay.ramani@citrix.com>
Imported-by: John Else <john.else@citrix.com>
Merge pull request #1859 from Zhengchai/HFX-1149

HFX-1149: pick creedence add a configurable idle timeout to stunnel #1779 to clearwater-sp1-lcm