Currently, validation of mappers (checking that cross-references are correct, limits on
field name lengths and object depths, multiple definitions, etc) is performed by the
MapperService. This means that any mapper-specific validation, for example that done
on the CompletionFieldMapper, needs to be called specifically from core server code,
and so we can't add validation to mappers that live in plugins.
This commit reworks the validation framework so that mapper-specific validation is
done on the Mapper itself. Mapper gets a new `validate(MappingLookup)`
method (already present on `MetadataFieldMapper` and now pulled up to the parent
interface), which is called from a new `DocumentMapper.validate()` method. All
the validation code currently living on `MapperService` moves either to individual
mapper implementations (FieldAliasMapper, CompletionFieldMapper) or into
`MappingLookup`, an altered `DocumentFieldMappers` which now knows about
object fields and can check for duplicate definitions, or into DocumentMapper
which handles soft limit checks.
b3ae5d26bd