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/rde/JSchSshSession.java

@@ -62,7 +62,7 @@ final class JSchSshSession implements Closeable {
       RdeUploadUrl url = RdeUploadUrl.create(uri);
       logger.info("Connecting to SSH endpoint: " + url);
       Session session = jsch.getSession(
-          url.getUser().or("domain-registry"),
+          url.getUser().orElse("domain-registry"),
           url.getHost(),
           url.getPort());
       if (url.getPass().isPresent()) {