api_service: Cache static pod info.
api_service: Cache static pod info.

Add a LRU cache to cache the static pod result, only the dynamic infos
(e.g. pod state, app state, etc.) will be read from the disk everytime.

achanda/rkt,kinvolk/rkt,ybubnov/rkt,akshaykarle/rkt,kinvolk/rkt,s-urbaniak/rkt,jakon89/rkt,lucasallan/rkt,intelsdi-x/rkt,alban/rkt,sstabellini/rkt,jonboulle/rkt,squeed/rkt,joshix/rkt,coreos/rkt,rkt/rkt,kinvolk/rkt,euank/rkt,nhlfr/rkt,Wilybald/rkt,Wilybald/rkt,akshaykarle/rkt,joshix/rkt,Shashwatsh/rkt,kinvolk/rkt,yifan-gu/rocket,lucasallan/rkt,yifan-gu/rkt,yifan-gu/rocket,shobhit85/rkt,joshix/rkt,cab105/rkt,cab105/rkt,sstabellini/rkt,squeed/rkt,yifan-gu/rkt,Shashwatsh/rkt,intelsdi-x/rkt,euank/rkt,yifan-gu/rkt,squeed/rkt,sstabellini/rkt,rkt/rkt,euank/rkt,s-urbaniak/rkt,yifan-gu/rocket,joshix/rkt,shobhit85/rkt,intelsdi-x/rkt,dgonyeo/rkt,coreos/rkt,coreos/rkt,alban/rkt,lucasallan/rkt,jakon89/rkt,jakon89/rkt,akshaykarle/rkt,nhlfr/rkt,s-urbaniak/rkt,rkt/rkt,jonboulle/rkt,ybubnov/rkt,nhlfr/rkt,dgonyeo/rkt,achanda/rkt,alban/rkt,rkt/rkt,Wilybald/rkt,lucasallan/rkt,achanda/rkt,cab105/rkt,dgonyeo/rkt,shobhit85/rkt,jonboulle/rkt,Shashwatsh/rkt,Wilybald/rkt,ybubnov/rkt