Improve DeltaFIFO function 'ListKeys'

In function ListKeys, it better to use ‘queue’ instead of 'items' to get all the keys.

Kubernetes-commit: f0ce8755287b1cad087b61abfcccd79c9f151828

tools/cache/delta_fifo_test.go

@@ -694,3 +694,23 @@ func TestDeltaFIFO_PopShouldUnblockWhenClosed(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDeltaFIFOListKeys(b *testing.B) {
+	f := NewDeltaFIFOWithOptions(DeltaFIFOOptions{KeyFunction: testFifoObjectKeyFunc})
+	const amount = 10000
+
+	for i := 0; i < amount; i++ {
+		f.Add(mkFifoObj(string([]rune{'a', rune(i)}), i+1))
+	}
+	for u := uint64(0); u < amount; u++ {
+		f.Add(mkFifoObj(string([]rune{'b', rune(u)}), u+1))
+	}
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			_ = f.ListKeys()
+		}
+	})
+	b.StopTimer()
+}