Break circular dependency between core and util (#379)

* Break circular dependency between core and util

Created a new :common project and moved a minimum
number of classes to break the circular dependency
between the two projects. This gets rid of the
gradle lint dependency warnings.

Also separated api classes and testing helpers into
separate source sets in :common so that testing
classes may be restricted to test configurations.

common/src/main/java/google/registry/util/Sleeper.java

@@ -0,0 +1,45 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.util;
+
+import javax.annotation.concurrent.ThreadSafe;
+import org.joda.time.ReadableDuration;
+
+/**
+ * An object which accepts requests to put the current thread to sleep.
+ *
+ * @see SystemSleeper
+ * @see google.registry.testing.FakeSleeper
+ */
+@ThreadSafe
+public interface Sleeper {
+
+  /**
+   * Puts the current thread to sleep.
+   *
+   * @throws InterruptedException if this thread was interrupted
+   */
+  void sleep(ReadableDuration duration) throws InterruptedException;
+
+  /**
+   * Puts the current thread to sleep, ignoring interrupts.
+   *
+   * <p>If {@link InterruptedException} was caught, then {@code Thread.currentThread().interrupt()}
+   * will be called at the end of the {@code duration}.
+   *
+   * @see com.google.common.util.concurrent.Uninterruptibles#sleepUninterruptibly
+   */
+  void sleepUninterruptibly(ReadableDuration duration);
+}