Make type in MappingMetadata private

[romseygeek]

We still have types hanging around internally in mappings, and it can be
a bit confusing trying to work out when they are needed and when not. This
commit moves type handling entirely within MappingMetadata, and removes
it from the public methods on MapperService.

This change means that system index descriptors need to explicitly wrap
their mapping values in a _doc type. Most of them already appear to do
this.