graphql: support existing gqlschema nodes without xid

[abhimanyusinghgaur]

This PR adds support for adding xid to existing gqlschema nodes as it was not taken into consideration in #5054.

Fixes #GRAPHQL-417.

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This change is 

[pawanrawal]

Is this a bugfix? If yes, then is it possible to add a testcase for this?

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

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graphql/admin/admin.go, line 472 at r1 (raw file):

	qry := &gql.GraphQuery{
		Attr: existingSchemaVar,

It would be helpful to write down the final query in comments that would be built here.

[abhimanyusinghgaur]

Added a testcase which checks that the GqlSchema node always has dgraph.graphql.xid with value dgraph.graphql.schema after graphql layer has started properly.

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @MichaelJCompton and @pawanrawal)

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graphql/admin/admin.go, line 472 at r1 (raw file):

Previously, pawanrawal (Pawan Rawal) wrote…

It would be helpful to write down the final query in comments that would be built here.

Done.

[pawanrawal]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MichaelJCompton)

[MichaelJCompton]

Yeah, this kind of thing might be hard to test in our testing setup ... ideally, we'd have some way where we could roll through an update from one version to the next. So I guess you could have a test with a 20.03.1 version, then restart on a 20.03.3 and see if it all updates smoothly. I don't think it's worth building that out just for this case. But I think it's a test we should have in general that runs from a 20.03.x to a 20.03.x+1 cause that gives us an assurance that a customer can upgrade smoothly.

Also, merging this, means we are committing to fixing this #5235 asap.

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MichaelJCompton)