layeredcache: add Stop() and fix races in tests

worker goroutine running concurrently with tests would cause data race errors
when running tests with -race enabled.

secondarycache_test.go

@@ -96,10 +96,10 @@ func (_ SecondaryCacheTests) TrackerDoesNotCleanupHeldInstance() {
 	sCache := cache.GetOrCreateSecondaryCache("0")
 	item := sCache.TrackingGet("a")
 	time.Sleep(time.Millisecond * 10)
-	cache.gc()
+	gcLayeredCache(cache)
 	Expect(cache.Get("0", "a").Value()).To.Equal(0)
 	Expect(cache.Get("1", "a")).To.Equal(nil)
 	item.Release()
-	cache.gc()
+	gcLayeredCache(cache)
 	Expect(cache.Get("0", "a")).To.Equal(nil)
 }