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/server/TestServer.java

@@ -121,7 +121,7 @@ public final class TestServer {
    * main event loop, for post-request processing.
    */
   public void ping() {
-    requestQueue.add(new FutureTask<Void>(doNothing(), null));
+    requestQueue.add(new FutureTask<>(doNothing(), null));
   }
 
   /** Stops the HTTP server. */