grpcproxy, etcdmain, integration: add close channel to kv proxy

ccache launches goroutines that need to be explicitly stopped.

Fixes #7158

proxy/grpcproxy/cache/store.go

@@ -39,6 +39,7 @@ type Cache interface {
 	Get(req *pb.RangeRequest) (*pb.RangeResponse, error)
 	Compact(revision int64)
 	Invalidate(key []byte, endkey []byte)
+	Close()
 }
 
 // keyFunc returns the key of an request, which is used to look up in the cache for it's caching response.
@@ -58,6 +59,8 @@ func NewCache(maxCacheEntries int) Cache {
 	}
 }
 
+func (c *cache) Close() { c.lru.Stop() }
+
 // cache implements Cache
 type cache struct {
 	mu  sync.RWMutex