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/request/auth/UserAuthInfo.java

@@ -16,7 +16,7 @@ package google.registry.request.auth;
 
 import com.google.appengine.api.users.User;
 import com.google.auto.value.AutoValue;
-import com.google.common.base.Optional;
+import java.util.Optional;
 
 /** Extra information provided by the authentication mechanism about the user. */
 @AutoValue
@@ -39,7 +39,7 @@ public abstract class UserAuthInfo {
 
   public static UserAuthInfo create(
       User user, boolean isUserAdmin) {
-    return new AutoValue_UserAuthInfo(user, isUserAdmin, Optional.<OAuthTokenInfo>absent());
+    return new AutoValue_UserAuthInfo(user, isUserAdmin, Optional.<OAuthTokenInfo>empty());
   }
 
   public static UserAuthInfo create(