Only allow one type on 6.0 indices

[jpountz]

This adds the index.mapping.single_type setting, which enforces that indices
have at most one type when it is true. The default value is true for 6.0+ indices
and false for old indices.

Most of the change is about fixing tests that created multiple types to either
create a single _type instead or set index.mapping.single_type to false
(eg. for parent/child tests, which still require multiple types).

Relates #15613

[jpountz]

Note to reviewers: the PR is a bit large but you can focus on changes in MapperService.java, TypeFieldMapper.java, MapperServiceTests.java and TypeFieldMapperTests.java. Other changes are just about fixing tests so that they pass.

[jpountz]

@martijnvg @jimczi Maybe you could have a look since it is a bit related to #20257. What I have been doing for now with parent/child tests was just to enable the _type field in the tests.

[rjernst]

I think maybe having this on _type will be confusing for users, and that it would be cleaner (both for users, and code wise) for this to be an index setting. Having it be enabled/disabled on the _type field has a sense that one could have it set differently for different document types, but in fact the entire point is it must be set to true on all of them, or false on the one type.

[clintongormley]

@rjernst good point - i'm happy to go with an index setting

[jpountz]

I'll go with index.mapper.single_type if that works for everyone.

[jpountz]

Done. I actually went with index.mapping.single_type since we only had one setting prefixed by index.mapper while the other ones are prefixed by index.mapping.

[jimczi]

LGTM

[jpountz]

I will merge this PR with a default value of false for now to give people who directly test against master a grace period they can use to set index.mapping.single_type on indices that still use multiple types.

[jpountz]

Removing the breaking label since the actual break comes in #24428.

[kukuxiahuni]

I have a question。 now my index and type used like this, bussiness_index/2017-11-12,bussiness_index/2017-11-13. just one index and more type.the type was every date string, and each type store the data of that day. when only one type in a index, how can i change my index structure??

[jpountz]

You cannot change the index structure in a single type. This kind of problem is typically solved by having time-based indices rather than types.

Note: please use discuss.elastic.co to ask questions rather than Github.