Both top level and inline inner hits are now covered by InnerHitBuilder.
Although there are differences between top level and inline inner hits,
they now make use of the same builder logic.
The parsing of top level inner hits slightly changed to be more readable.
Before the nested path or parent/child type had to be specified as encapsuting
json object, now these settings are simple fields. Before this was required
to allow streaming parsing of inner hits without missing contextual information.
Once some issues are fixed with inline inner hits (around multi level hierachy of inner hits),
top level inner hits will be deprecated and removed in the next major version.
Currently if you run an `exists` query on an object, it will resolve all sub
fields and create a disjunction for all those fields. However the `_field_names`
mapper indexes paths for objects so we could query object paths directly.
I also changed the query parser to reject `exists` queries if the `_field_names`
field is disabled since it would be a big performance trap.
Clinton Gormley
Martijn van Groningen
Isabel Drost-Fromm
Adrien Grand