[Code] Control whether MapperService docMapper iterator should contain DEFAULT_MAPPING

At the moment one can iterate the MapperService to go through all document mappers. This includes the document mapper of DEFAULT_MAPPING, which may be surprising and lead to unintended results. This commit removes the Iterable implementation and add a docMappers method that asks the caller to make an explicit choice
can we have a static final inner class for this predicate? it seems static?