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/backup/DeleteOldCommitLogsAction.java

@@ -143,12 +143,7 @@ public final class DeleteOldCommitLogsAction implements Runnable {
       //
       // We want to make sure we retry any load individually to reduce the chance of the entire
       // shard failing, hence we wrap it in a transactNew.
-      Object object = ofy().transactNew(new Work<Object>() {
-        @Override
-        public Object run() {
-          return ofy().load().key(key).now();
-        }
-      });
+      Object object = ofy().transactNew(() -> ofy().load().key(key).now());
       checkNotNull(object, "Received a key to a missing object. key: %s", key);
       checkState(
           object instanceof EppResource,