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

java/google/registry/rdap/RdapModule.java

@@ -14,12 +14,12 @@
 
 package google.registry.rdap;
 
-import com.google.common.base.Optional;
 import dagger.Module;
 import dagger.Provides;
 import google.registry.request.Parameter;
 import google.registry.request.RequestParameters;
 import java.net.InetAddress;
+import java.util.Optional;
 import javax.servlet.http.HttpServletRequest;
 
 /** Dagger module for the RDAP package. */