Support passing GraphQL schema to bulk loader.

[martinmr]

Support passing a GraphQL schema file to the bulk loader. The bulk loader will generate
the triples relevant triples expected by Dgraph.

Related to DGRAPH-1283

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This change is 

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[martinmr]

I tested this manually and it works. I will work on automated tests first thing next week.

[martinmr]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @manishrjain, @MichaelJCompton, @pawanrawal, and @vvbalaji-dgraph)

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dgraph/cmd/bulk/reduce.go, line 61 at r1 (raw file):

	x.AssertTrue(len(r.opt.shardOutputDirs) == r.opt.ReduceShards)

	r.dbs = make([]*badger.DB, r.opt.ReduceShards)

Ignore the changes to this file. I thought I needed to change the reduce phase at first. I will revert these changes.

[vvbalaji-dgraph]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @manishrjain, @martinmr, @MichaelJCompton, @pawanrawal, and @vvbalaji-dgraph)

a discussion (no related file):

I will work on automated tests first thing next week.
:lgtm

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[pawanrawal]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @manishrjain, @martinmr, and @MichaelJCompton)

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dgraph/cmd/bulk/loader.go, line 275 at r1 (raw file):

	x.Check(err)
	if filepath.Ext(ld.opt.GqlSchemaFile) == ".gz" {
		r, err = gzip.NewReader(r)

Do you need to defer close this reader too?

[martinmr]

Reviewable status: 2 of 4 files reviewed, 2 unresolved discussions (waiting on @manishrjain, @MichaelJCompton, @pawanrawal, and @vvbalaji-dgraph)

a discussion (no related file):

Previously, vvbalaji-dgraph (V V Balaji) wrote…

I will work on automated tests first thing next week.
:lgtm

Done. I have added the tests.

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dgraph/cmd/bulk/loader.go, line 275 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

Do you need to defer close this reader too?

io.Reader does not have a close method. I think it's safe. The file reader is being closed.