[7.x](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: 2021-09-22T14:08:17.485+0000

• Duration: 103 min 50 sec

• Commit: 86c4ac0

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Test	Results
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Passed	7060
Skipped	24
Total	7084

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Test	Results
Failed	0
Passed	7060
Skipped	24
Total	7084

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