ref[SingleCache]: move SingleCache to scheduler

scheduler/src/main/java/com/zfoo/scheduler/util/SingleCache.java

@@ -11,9 +11,7 @@
  * See the License for the specific language governing permissions and limitations under the License.
  */
 
-package com.zfoo.net.util;
-
-import com.zfoo.scheduler.util.TimeUtils;
+package com.zfoo.scheduler.util;
 
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Supplier;