ISPN-4528 Genericize remaining M/R internal classes

Master only. See https://issues.jboss.org/browse/ISPN-4528 for more details. 
This should work
```
public static class DeltaListExternalizer<DE> extends AbstractExternalizer<DeltaList<DE>> {

      private static final long serialVersionUID = 5859147782602054109L;

      @Override
      public void writeObject(ObjectOutput output, DeltaList<DE> list) throws IOException {
         output.writeObject(list.deltas);
      }

      @Override
      public DeltaList readObject(ObjectInput input) throws IOException, ClassNotFoundException {
         return new DeltaList((List<DE>) input.readObject());
      }

      @Override
      public Integer getId() {
         return Ids.DELTA_MAPREDUCE_LIST_ID;
      }

      @Override
      public Set<Class<? extends DeltaList<DE>>> getTypeClasses() {
         return Util.<Class<? extends DeltaList<DE>>>asSet(DeltaList.class);
      }
   }
```