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/auth/AuthResult.java

@@ -17,7 +17,7 @@ package google.registry.request.auth;
 import static com.google.common.base.Preconditions.checkNotNull;
 
 import com.google.auto.value.AutoValue;
-import com.google.common.base.Optional;
+import java.util.Optional;
 import javax.annotation.Nullable;
 
 /**
@@ -37,14 +37,14 @@ public abstract class AuthResult {
   }
 
   public static AuthResult create(AuthLevel authLevel) {
-    return new AutoValue_AuthResult(authLevel, Optional.<UserAuthInfo>absent());
+    return new AutoValue_AuthResult(authLevel, Optional.<UserAuthInfo>empty());
   }
 
   public static AuthResult create(AuthLevel authLevel, @Nullable UserAuthInfo userAuthInfo) {
     if (authLevel == AuthLevel.USER) {
       checkNotNull(userAuthInfo);
     }
-    return new AutoValue_AuthResult(authLevel, Optional.fromNullable(userAuthInfo));
+    return new AutoValue_AuthResult(authLevel, Optional.ofNullable(userAuthInfo));
   }
 
   /**

java/google/registry/request/auth/RequestAuthenticator.java

@@ -18,11 +18,11 @@ import static com.google.common.base.Preconditions.checkArgument;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.annotations.VisibleForTesting;
-import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Ordering;
 import com.google.errorprone.annotations.Immutable;
 import google.registry.util.FormattingLogger;
+import java.util.Optional;
 import javax.inject.Inject;
 import javax.servlet.http.HttpServletRequest;
 
@@ -117,14 +117,14 @@ public class RequestAuthenticator {
       case APP:
         if (!authResult.isAuthenticated()) {
           logger.warning("Not authorized; no authentication found");
-          return Optional.absent();
+          return Optional.empty();
         }
         break;
       case USER:
         if (authResult.authLevel() != AuthLevel.USER) {
           logger.warning("Not authorized; no authenticated user");
           // TODO(mountford): change this so that the caller knows to return a more helpful error
-          return Optional.absent();
+          return Optional.empty();
         }
         break;
     }
@@ -132,7 +132,7 @@ public class RequestAuthenticator {
       case IGNORED:
         if (authResult.authLevel() == AuthLevel.USER) {
           logger.warning("Not authorized; user policy is IGNORED, but a user was found");
-          return Optional.absent();
+          return Optional.empty();
         }
         break;
       case PUBLIC:
@@ -142,7 +142,7 @@ public class RequestAuthenticator {
         if (authResult.userAuthInfo().isPresent()
             && !authResult.userAuthInfo().get().isUserAdmin()) {
           logger.warning("Not authorized; user policy is ADMIN, but the user was not an admin");
-          return Optional.absent();
+          return Optional.empty();
         }
         break;
     }

java/google/registry/request/auth/UserAuthInfo.java

@@ -16,7 +16,7 @@ package google.registry.request.auth;
 
 import com.google.appengine.api.users.User;
 import com.google.auto.value.AutoValue;
-import com.google.common.base.Optional;
+import java.util.Optional;
 
 /** Extra information provided by the authentication mechanism about the user. */
 @AutoValue
@@ -39,7 +39,7 @@ public abstract class UserAuthInfo {
 
   public static UserAuthInfo create(
       User user, boolean isUserAdmin) {
-    return new AutoValue_UserAuthInfo(user, isUserAdmin, Optional.<OAuthTokenInfo>absent());
+    return new AutoValue_UserAuthInfo(user, isUserAdmin, Optional.<OAuthTokenInfo>empty());
   }
 
   public static UserAuthInfo create(