* Replace go-cache with golang-lru

This fix replace go-cache with golang-lru, as is specified in 335.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Move cache initialization to setup

This commit move cache initialization to setup in dnssec middleware.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>