BUG/MINOR: connection: "connection:close" header added despite 'close-spread-time'

BUG/MINOR: connection: "connection:close" header added despite 'close-spread-time'

Despite what the 'close-spread-time' option should do, the
'connection:close' header was always added to HTTP responses during
soft-stops even with a soft-stop window defined.
This patch adds the proper random based closing to HTTP connections
during a soft-stop (based on the time left in the soft close window).

It should be backported to 2.5 once 'MEDIUM: global: Add a
"close-spread-time" option to spread soft-stop on time window' is
backported as well.