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

java/google/registry/flows/session/LoginFlow.java

@@ -17,7 +17,6 @@ package google.registry.flows.session;
 import static com.google.common.collect.Sets.difference;
 import static google.registry.util.CollectionUtils.nullToEmpty;
 
-import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableSet;
 import google.registry.flows.EppException;
 import google.registry.flows.EppException.AuthenticationErrorClosingConnectionException;
@@ -42,6 +41,7 @@ import google.registry.model.eppinput.EppInput.Services;
 import google.registry.model.eppoutput.EppResponse;
 import google.registry.model.registrar.Registrar;
 import google.registry.util.FormattingLogger;
+import java.util.Optional;
 import java.util.Set;
 import javax.inject.Inject;