(fix/dgraph): Fix facets response with normalize

[ashish-goswami]

Fixes: #5241
Fixes: DGRAPH-1670

This PR fixes issue with facets when it is retrieved in a query containing @normalize directive.
While forming @normalize response, we flatten a fastJsonNode and make its grand children,
direct children of it. This should be valid in all of cases except when fastJsonNode is parent of
facets nodes. For example consider below data:

<0x1> <name> "Alice" .
<0x1> <friend> "Bob" (from="college") .
<0x1> <friend> "Roman" (from="school") .

Also consider below query:

q(func: uid(0x1)) @normalize {
    name: name
    friend: friends @facets
}

Expected response is:

{
  "data": {
    "q": [
      {
        "name": "Alice",
        "friends|from": {
          "0": "college",
          "1": "school"
        },
        "friends": [
          "Bob",
          "Roman"
        ]
      }
    ]
  }
}

But actual response is:

{
  "data": {
    "q": [
      {
        "0": "college",
        "1": "school",
        "friends": [
          "Bob",
          "Roman"
        ],
        "name": "Alice"
      }
    ]
  }
}

Its happening because we are flattening facet parent node friends|from as well which have node
"0" and "1" as children.
We are solving it by having extra information in the node if it is a facets parent.

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This change is 

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[pawanrawal]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @ashish-goswami)

---

query/outputnode.go, line 169 at r1 (raw file):

// 3. List => Stores boolean value, true if this node is part of list.
// 4. FacetsParent => Stores boolean value, true if this node is a facetsParent. facetsParent is
//    node which is parent for facets values for a scalar list predicaite. Eg: node "country|city"

city|country

predicate

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query/outputnode.go, line 662 at r1 (raw file):

		// Here we are counting all non-scalar attributes of fj. If there are any such
		// attributes, we will flatten it, otherwise we will return all attributes.
		// We should only consider those nodes for falttening which have children and are not

flatenning

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query/query_facets_test.go, line 2118 at r1 (raw file):

}

func TestFacetValueListPredicateWithNormalize(t *testing.T) {

TestFacetValuePredicateWithNormalize

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query/query_facets_test.go, line 2121 at r1 (raw file):

	populateClusterWithFacets()
	query := `{
		q(func: uid(1, 12000)) @normalize {

Also add a test where value is nested like

func: uid(1, 12000) @normalize {
  name
  friends {
    name @facets 
  }
}

[martinmr]

Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @ashish-goswami)

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query/outputnode.go, line 263 at r1 (raw file):

(

I don't think you need the outer parenteses

[vvbalaji-dgraph]

lgtm. Thanks for adding the tests