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.

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

javatests/google/registry/testing/DatastoreHelper.java

@@ -103,6 +103,7 @@ import google.registry.model.transfer.TransferStatus;
 import google.registry.tmch.LordnTask;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Set;
 import javax.annotation.Nullable;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
@@ -678,7 +679,7 @@ public class DatastoreHelper {
   }
 
   /** Assert that the expected billing events set is exactly the one found in the fake Datastore. */
-  public static void assertBillingEvents(ImmutableSet<BillingEvent> expected) throws Exception {
+  public static void assertBillingEvents(Set<BillingEvent> expected) throws Exception {
     assertBillingEventsEqual(getBillingEvents(), expected);
   }