config service: fix latest config cache timeout
config service: fix latest config cache timeout

There should be timeout for a cached value of a latest config.

TBR=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1130263004

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py