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/braintree/BraintreeRegistrarSyncer.java

@@ -22,10 +22,10 @@ import com.braintreegateway.Customer;
 import com.braintreegateway.CustomerRequest;
 import com.braintreegateway.Result;
 import com.braintreegateway.exceptions.NotFoundException;
-import com.google.common.base.Optional;
 import com.google.common.base.VerifyException;
 import google.registry.model.registrar.Registrar;
 import google.registry.model.registrar.RegistrarContact;
+import java.util.Optional;
 import javax.inject.Inject;
 
 /** Helper for creating Braintree customer entries for registrars. */
@@ -89,7 +89,7 @@ public class BraintreeRegistrarSyncer {
         return Optional.of(contact);
       }
     }
-    return Optional.absent();
+    return Optional.empty();
   }
 
   private boolean doesCustomerExist(String id) {