Merging mappings ensures that fields are used consistently across mapping types. Disabling norms for a specific field in one mapping type for example also disables norms for the same field in other mapping types of that index. The logic that ensures this while merging mappings currently always creates a fresh document mapper for all existing mapping types, even if no change occurred. Creating such a fresh document mapper does not come for free though as it involves recompressing the source. Making a mapping change to one type of an index with 100 types will thus re-serialize and recompress all 100 types, independent of any changes made to those types.

This commit fixes the update logic to only create a new DocumentMapper if a field type actually changes.