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/flows/TlsCredentialsTest.java

@@ -15,13 +15,13 @@
 package google.registry.flows;
 
 import static com.google.common.truth.Truth.assertThat;
+import static com.google.common.truth.Truth8.assertThat;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
 import google.registry.request.HttpException.BadRequestException;
 import google.registry.testing.ExceptionRule;
 import javax.servlet.http.HttpServletRequest;
-
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;