Deprecate org.infinispan.protostream.Message interface and introduce a more sensible way of handling unknown fields

org.infinispan.protostream.Message mechanism for supporting unknown fields is a bit too intrusive because it forces users to implement our interface in their domain model classes.

A better approach would be to have a similar interface implemented by the marshaller object instead.

public interface UnknownFieldSetHandler<T> {

   UnknownFieldSet getUnknownFieldSet(T message);

   void setUnknownFieldSet(T message, UnknownFieldSet unknownFieldSet);
}