Limit UIDs per variable in upsert

[mangalaman93]

fixes #4021

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This change is 

[golangcibot]

G404: Use of weak random number generator (math/rand instead of crypto/rand) (from gosec)

[martinmr]

Reviewed 2 of 13 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @danielmai, @mangalaman93, @manishrjain, @MichaelJCompton, and @pawanrawal)

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dgraph/cmd/alpha/upsert_test.go, line 2559 at r2 (raw file):

}

// This test is supposed to be ignored by the CI

Maybe add a comment on why it should be ignored.

[manishrjain]

Got a comment.

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @danielmai, @mangalaman93, @martinmr, @MichaelJCompton, and @pawanrawal)

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edgraph/server.go, line 996 at r3 (raw file):

		// don't do bad things to alpha and mutation doesn't become too big.
		if len(v.Uids.Uids) > 1e6 {
			return resp, errors.Errorf("variable [%v] has too many UIDs (>1m)", name)

"has over million UIDs. Please narrow .. down."

[pawanrawal]

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @danielmai, @mangalaman93, @martinmr, @MichaelJCompton, and @pawanrawal)