beater: move event ACKer from publisher to beater

[axw]

Motivation/summary

In the future we will start and stop multiple publishers.
To avoid blocking the publisher.Stop method unnecessarily
during a config reload, move event ACKing to beater.

How to test these changes

1. Run APM Server with a shutdown timeout configured
2. Stop Elasticsearch
3. Send some events
4. Shutdown APM Server, check that the shutdown timeout is honoured

Related issues

Preparation for #4376

[apmmachine]

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Build stats

• Build Cause: Pull request #4617 opened

• Start Time: 2021-01-13T04:58:35.617+0000

• Duration: 45 min 58 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4607
Skipped	124
Total	4731

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