[7.15](backport #27981) [Elastic Agent] Fix lazy acker to only add new actions to the batch

[mergify]

This is an automatic backport of pull request #27981 done by Mergify.

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• Start Time: 2021-09-22T14:09:27.745+0000

• Duration: 96 min 44 sec

• Commit: c70dc8f

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Test	Results
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Passed	6988
Skipped	24
Total	7012

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Test	Results
Failed	0
Passed	6988
Skipped	24
Total	7012

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