Bulk indexer: Follow convention and increment waitgroup in loop before creating goroutines

[HaraldNordgren]

The wg Delta should generally be added before relevant go-routines are launched. Reference: https://pkg.go.dev/sync#WaitGroup.Add

This also aligns the code with our other uses of waitgroups.

Closes https://github.com/elastic/go-elasticsearch/issues/796.

[HaraldNordgren]

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