Imports of package cache

PathSynopsis
github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes
github.com/coreos/etcd/etcdserver/etcdserverpbPackage etcdserverpb is a generated protocol buffer package.
github.com/golang/groupcache/lruPackage lru implements an LRU cache.
syncPackage sync provides basic synchronization primitives such as mutual exclusion locks.