Support running indexing in background.

[danielmai]

As of Dgraph v20.03.0, indexing can be run in the background: https://dgraph.io/docs/query-language/#indexes-in-background

As of v20.03.1 in-progress indexing tasks can be tracked via /health ("ongoing" and "indexing"): https://dgraph.io/docs/deploy/#querying-health

[minhaj-shakeel]

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