Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

javatests/google/registry/batch/MapreduceEntityCleanupActionTest.java

@@ -110,7 +110,7 @@ public class MapreduceEntityCleanupActionTest
                 .setInput(input)
                 .setMapper(new TestMapper())
                 .setReducer(new TestReducer())
-                .setOutput(new InMemoryOutput<String>())
+                .setOutput(new InMemoryOutput<>())
                 .setNumReducers(2)
                 .build(),
             new MapReduceSettings.Builder().setWorkerQueueName(QUEUE_NAME).build());