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

javatests/google/registry/module/backend/BUILD

@@ -18,8 +18,9 @@ java_library(
         "//java/google/registry/request",
         "//java/google/registry/util",
         "//javatests/google/registry/testing",
-        "//third_party/java/truth",
         "@com_google_guava",
+        "@com_google_truth",
+        "@com_google_truth_extensions_truth_java8_extension",
         "@javax_servlet_api",
         "@junit",
         "@org_mockito_all",