Add Runnable overrides to ease use of Java 8 language features

Runnable and Callable are both @FunctionalInterfaces. The difference is
that Callable requires a return value whereas Runnable does not, so in
situations where we don't care about a return value, rather than having to
add an unnecessary 'return null;' at the end of the lambda, we can simply
use a non-returning Runnable instead.

Unfortunately, owing to legacy reasons, Runnable is not declared to throw
checked exceptions whereas Callable is, so in situations where checked
exceptions are thrown we still need to have a 'return null;' call at the
end.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172935400

java/google/registry/export/SyncGroupMembersAction.java

@@ -27,7 +27,6 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Sets;
 import com.google.common.collect.Streams;
-import com.googlecode.objectify.VoidWork;
 import google.registry.config.RegistryConfig.Config;
 import google.registry.groups.GroupsConnection;
 import google.registry.groups.GroupsConnection.Role;
@@ -134,12 +133,7 @@ public final class SyncGroupMembersAction implements Runnable {
         new ImmutableMap.Builder<>();
     for (final Registrar registrar : dirtyRegistrars) {
       try {
-        retrier.callWithRetry(
-            () -> {
-              syncRegistrarContacts(registrar);
-              return null;
-            },
-            RuntimeException.class);
+        retrier.callWithRetry(() -> syncRegistrarContacts(registrar), RuntimeException.class);
         resultsBuilder.put(registrar, Optional.<Throwable>empty());
       } catch (Throwable e) {
         logger.severe(e, e.getMessage());
@@ -171,11 +165,7 @@ public final class SyncGroupMembersAction implements Runnable {
         registrarsToSave.add(result.getKey().asBuilder().setContactsRequireSyncing(false).build());
       }
     }
-    ofy().transactNew(new VoidWork() {
-      @Override
-      public void vrun() {
-          ofy().save().entities(registrarsToSave.build());
-      }});
+    ofy().transactNew(() -> ofy().save().entities(registrarsToSave.build()));
     return errors;
   }