Reject index requests that contain metadata fields in the request body.

[martijnvg]

Metadata fields (_routing, _parent, _ttl) can also be specified in the request body and in certain scenarios they don't get indexed (just stored in the source).

Example (ttl is not configured):

{
"_ttl" : 1
}

A term query on _ttl field would fail in the case, this can be confusing, especially when the ttl feature isn't used at all.

In the case that metadata fields are in the request body and these fields don't get indexed then this index request should fail.

[s1monw]

@martijnvg are you gonna work on this?

[clintongormley]

Close in favour of #6736