Rewrite SourceToParse with resolved docType

[dnhatn]

We introduce a typeless API in #35790 where we translate the default docType "_doc" to the user-defined docType. However, we do not rewrite the SourceToParse with the resolved docType. This leads to a situation where we have two translog operations for the same document with different types:

• prvOp [Index{id='9LCpwGcBkJN7eZxaB54L', type='_doc', seqNo=1, primaryTerm=1, version=1, autoGeneratedIdTimestamp=1545125562123}]

• newOp [Index{id='9LCpwGcBkJN7eZxaB54L', type='not_doc', seqNo=1, primaryTerm=1, version=1, autoGeneratedIdTimestamp=-1}]

Closes #36769

[elasticmachine]

Pinging @elastic/es-search

[dnhatn]

We may have a BWC issue with the Typeless API if the primary is on 7.0 while replicas on 6.x. I'll test that scenario and post the result.

[markharwood]

We may have a BWC issue with the Typeless API if the primary is on 7.0 while replicas on 6.x. I'll test that scenario and post the result.

I'm working on a PR for the bulk API which just hit this. @mayya-sharipova recently changed IndexRequest to default type to _doc but in my PR I need to default it to null so that I know a choice hasn't been made by the user and can inherit any default "global" type given to the containing BulkRequest. I found serializing a null type from 7.x to 6.x nodes was causing a null pointer so I had to make sure that the serialized type was defaulted to "_doc" if null.

The above detail may not change the status quo on the receiving 6.x end which you're tackling here (provided IndexRequest = _doc but mapping = custom type) but it may be useful to know the related work going on and the CI reproduce line that uncovered my issue: 2db6753

[jpountz]

The change looks good to me.

Yannick talked to me about this issue earlier today and I do agree that we need to find a more robust way to make typeless APIs work on indices that still have a type whose name is different from _doc.

[dnhatn]

run Gradle build tests 1

[dnhatn]

Thanks @jpountz for reviewing and @markharwood for the heads-up.