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/request/Router.java

@@ -18,12 +18,12 @@ import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Throwables.throwIfUnchecked;
 
 import com.google.common.base.Function;
-import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableSortedMap;
 import com.google.common.collect.Ordering;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.util.Map;
+import java.util.Optional;
 import java.util.TreeMap;
 
 /**
@@ -62,7 +62,7 @@ final class Router {
         return Optional.of(floor.getValue());
       }
     }
-    return Optional.absent();
+    return Optional.empty();
   }
 
   static ImmutableSortedMap<String, Route> extractRoutesFromComponent(Class<?> componentClass) {