fix(export): Fix facet export of reference type postings to JSON format

[ahsanbarkati]

Consider this data:

_:blank-0 <name> "Carol" (initial="C") .
_:blank-0 <dgraph.type> "Person" .
_:blank-0 <friend> _:blank-1 (close="yes") .
_:blank-1 <name> "Daryl" .
_:blank-1 <dgraph.type> "Person" .

Currently export generates this JSON for it:

[
  {"uid":"0x1","name":"Carol","name|initial":"C"},
  {"uid":"0x2","name":"Daryl"},
  {"uid":"0x1","friend":[{"uid":"0x2"}],"friend|close":"yes"},
  {"uid":"0x1","dgraph.type":"Person"},
  {"uid":"0x2","dgraph.type":"Person"}
]

The above generated JSON is not right. In {"uid":"0x1","friend":[{"uid":"0x2"}],"friend|close":"yes"}, , we do not keep a mapping about the facet belongs to which uid in the list. And when this data is loaded, the facets gets lost.

After this change we will get this:

[
  {"uid":"0x1","name":"Carol","name|initial":"C"},
  {"uid":"0x2","name":"Daryl"},
  {"uid":"0x1","friend":[{"uid":"0x2", "friend|close":"yes"}]},
  {"uid":"0x1","dgraph.type":"Person"},
  {"uid":"0x2","dgraph.type":"Person"}
]

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This change is 

[pawanrawal]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)