Rewrite ServerSecret caching and accessor logic

I'm working on some changes to XsrfTokenManager (b/35388772) and ServerSecret
was crufty enough that I ended up rewriting it.  Now it uses a LoadingCache
with a transaction instead of needlessly race-condition-y static init logic.

It also now supports retrieving its value as either a UUID (the old format
used by XsrfTokenManager) or a byte[].  The latter is more flexible and can
be directly used with HMAC which the new XsrfTokenManager format will employ.

And lastly, I added tests.  In addition, I tested this code on alpha and
verified appropriate operation (XSRF tokens still work from the console and
from regtool; if you remove ServerSecret from datastore and memcache, it
persists a new one).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148931620

javatests/google/registry/model/server/ServerSecretTest.java

@@ -0,0 +1,80 @@
+// 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.model.server;
+
+import static com.google.common.truth.Truth.assertThat;
+import static google.registry.model.ofy.ObjectifyService.ofy;
+
+import google.registry.model.ofy.RequestCapturingAsyncDatastoreService;
+import google.registry.testing.AppEngineRule;
+import java.util.UUID;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link ServerSecret}. */
+@RunWith(JUnit4.class)
+public class ServerSecretTest {
+  @Rule
+  public final AppEngineRule appEngine = AppEngineRule.builder()
+      .withDatastore()
+      .build();
+
+  @Before
+  public void before() {
+    ServerSecret.resetCache();
+  }
+
+  @Test
+  public void testGet_bootstrapping_savesSecretToDatastore() throws Exception {
+    ServerSecret secret = ServerSecret.get();
+    assertThat(secret).isNotNull();
+    assertThat(ofy().load().entity(new ServerSecret()).now()).isEqualTo(secret);
+  }
+
+  @Test
+  public void testGet_existingSecret_returned() throws Exception {
+    ServerSecret secret = ServerSecret.create(123, 456);
+    ofy().saveWithoutBackup().entity(secret).now();
+    assertThat(ServerSecret.get()).isEqualTo(secret);
+    assertThat(ofy().load().entity(new ServerSecret()).now()).isEqualTo(secret);
+  }
+
+  @Test
+  public void testGet_cachedSecret_returnedWithoutDatastoreRead() throws Exception {
+    int numInitialReads = RequestCapturingAsyncDatastoreService.getReads().size();
+    ServerSecret secret = ServerSecret.get();
+    int numReads = RequestCapturingAsyncDatastoreService.getReads().size();
+    assertThat(numReads).isGreaterThan(numInitialReads);
+    assertThat(ServerSecret.get()).isEqualTo(secret);
+    assertThat(RequestCapturingAsyncDatastoreService.getReads()).hasSize(numReads);
+  }
+
+  @Test
+  public void testAsUuid() throws Exception {
+    UUID uuid = ServerSecret.create(123, 456).asUuid();
+    assertThat(uuid.getMostSignificantBits()).isEqualTo(123);
+    assertThat(uuid.getLeastSignificantBits()).isEqualTo(456);
+  }
+
+  @Test
+  public void testAsBytes() throws Exception {
+    byte[] bytes = ServerSecret.create(123, 0x456).asBytes();
+    assertThat(bytes)
+        .isEqualTo(new byte[] {0, 0, 0, 0, 0, 0, 0, 123, 0, 0, 0, 0, 0, 0, 0x4, 0x56});
+  }
+}