Switch from Guava Optionals to Java 8 Optionals

This was a surprisingly involved change. Some of the difficulties included
java.util.Optional purposely not being Serializable (so I had to move a
few Optionals in mapreduce classes to @Nullable) and having to add the Truth
Java8 extension library for assertion support.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171863777

javatests/google/registry/tools/DefaultRequestFactoryModuleTest.java

@@ -15,6 +15,7 @@
 package google.registry.tools;
 
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth8.assertThat;
 
 import com.google.api.client.auth.oauth2.Credential;
 import com.google.api.client.http.HttpRequest;