rgw: s/idle_timeout/default_idle_timeout/

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #90 from dachary/wip-8988-default-idle-timeout

rgw: s/idle_timeout/default_idle_timeout/

Reviewed-by: John Spray <john.spray@redhat.com>