feat(live): added upsert in live loader

[harshil-goel]

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This change is 

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[manishrjain]

Let's first narrow down why Dgraph was going OOM for upserts run via it. Think the debugging showed that the cache wasn't properly set up. So, do that first. And then see if you still need this change.

Overall, upserts in Dgraph should work and we should make them more robust.

Reviewable status: 0 of 7 files reviewed, 2 unresolved discussions (waiting on @harshil-goel, @manishrjain, and @vvbalaji-dgraph)

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dgraph/cmd/live/run.go, line 156 at r1 (raw file):

	flag.Bool("ludicrous_mode", false, "Run live loader in ludicrous mode (Should "+
		"only be done when alpha is under ludicrous mode)")
	flag.StringP("upsertPredicate", "U", "", "run in upsertPredicate mode. the value would be used to "+

100 chars.

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dgraph/cmd/live/run.go, line 255 at r1 (raw file):

	for _, i := range nqs {
		// taking hash as the value might contain invalid symbols
		ids[i.Subject] = fmt.Sprintf("u_%d", farm.Fingerprint64([]byte(i.Subject)))

Avoid fmt.Sprintf wherever possible. Perhaps strconv.Itoa.

[parasssh]

minor comments.

Reviewable status: 0 of 7 files reviewed, 5 unresolved discussions (waiting on @harshil-goel and @vvbalaji-dgraph)

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dgraph/cmd/live/run.go, line 245 at r2 (raw file):

}

func generateUidFunc(val string) string {

Put an example of the generated UidFunc in comments

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dgraph/cmd/live/run.go, line 253 at r2 (raw file):

}

func generateQuery(node, predicate, xid string) string {

Same here. Write an example in comments.

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dgraph/cmd/live/run.go, line 269 at r2 (raw file):

	// We form upsertPredicate query for each of the ids we saw in the request, along with
	// adding the corresponding xid to that uid. The mutation we added is only useful if the
	// uid doesn't exists.

if m.1234 exists, then the mutation still happens on the same m.1234? Is there a way to make it no-op so we dont incur the cost of additional mutation.

[jarifibrahim]

Related to DGRAPH-1181