Add FieldMappers before adding Mappers (used for parsing) and while holding the relevant's ObjectMapper mutex

Adding the mappers first could cause the wrong FieldMappers to be used during object parsing

Also:
Removed MergeContext.newFieldMappers, MergeContext.newObjectMappers and relatives as they are not used anymore.
Similarly removed MergeContext.addFieldDataChange & fieldMapperChanges as they were not used.

Closes #3844