[7.x](backport #319) Refactor bulk API to minimize allocations and increase speed.

[mergify]

This is an automatic backport of pull request #319 done by Mergify.

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[elasticmachine]

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Build stats

• Build Cause: Pull request #342 opened

• Start Time: 2021-05-12T12:38:46.320+0000

• Duration: 5 min 14 sec

• Commit: 186f3c9

Test stats 🧪

Test	Results
Failed	0
Passed	174
Skipped	0
Total	174

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