Consistency between `_search` and `_msearch`

[ofavre]

The _msearch API doesn't accept the same header parameters than _search's query parameters.
For instance version is both valid in the search body and query parameters for _search, but is ignored in the query parameters and header in _msearch.

MultiSearchRequest.add() should rely on RestSearchAction.parseSearchSource() or even RestSearchAction.parseSearchRequest() to parse the subrequests, for maximum compatibility.

IMHO, a clean bulk API should use the header parameters as query parameters on the target endpoint, which would imply that they keep in sync no matter the code changes in the target endpoints.

[kimchy]

yea, agreed, that would be nice to have

[clintongormley]

Should _search be implemented internally via _msearch, in which case the two would never go out of sync? @Christoph and @martijnvg what do you think?

[mayya-sharipova]

cc @elastic/es-search-aggs

[pedromatt]

Is this issue still active? I can't completely understand what is trying to be fixed and I'd like to work on it