Overwrite values for uid predicates

[martinmr]

When the warning to delete and re-add the value was removed, non-list uid
predicates were not being overwritten. This fixes this by iterating over the list
and replacing theexisting postings with a copy with the operation set to a delete.

It also adds a test to prevent this from happening again.

Fixes #4879

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This change is 

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[pawanrawal]

@martinmr could you please see why the CI is failing?

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @manishrjain and @martinmr)

[martinmr]

Fixed the failing tests.

Reviewable status: 1 of 2 files reviewed, 3 unresolved discussions (waiting on @golangcibot, @manishrjain, and @pawanrawal)

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posting/list.go, line 343 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

Error return value of l.iterate is not checked (from errcheck)

Done.

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posting/list.go, line 441 at r2 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

		pk.IsData() && mpost.Op == Set && mpost.PostingType == pb.Posting_REF

Done.

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posting/list.go, line 441 at r3 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

		pk.IsData() && mpost.Op == Set && mpost.PostingType == pb.Posting_REF

Done.

[manishrjain]

Reviewed 1 of 2 files at r1, 1 of 1 files at r5.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @golangcibot and @martinmr)

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posting/list.go, line 339 at r5 (raw file):

	if singleUidUpdate {
		// This handles the special case when adding a value to predicates of type uid.

We have to do this, because the UID for a uid predicate would not be MaxUint64, as is the case for other non-list scalars. So, we need to delete the previous whatever UID is there and set this one.

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posting/list.go, line 451 at r5 (raw file):

			hex.EncodeToString(l.key), mpost)
	}

Ludicrous mode return might have to be moved here.

[manishrjain]

Add a bunch of comments.

Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @golangcibot and @martinmr)

[martinmr]

Added more comments.

Reviewable status: 1 of 2 files reviewed, 5 unresolved discussions (waiting on @golangcibot and @manishrjain)

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posting/list.go, line 339 at r5 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

We have to do this, because the UID for a uid predicate would not be MaxUint64, as is the case for other non-list scalars. So, we need to delete the previous whatever UID is there and set this one.

Done.

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posting/list.go, line 451 at r5 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Ludicrous mode return might have to be moved here.

Done.