rename queue.Batch.ACK -> queue.Batch.Done

[faec]

What does this PR do?

Rename the ACK function in the queue.Batch interface to Done. This function is the only way to finish handling a batch, and must be called regardless of whether the events were acked or not. Its only effect is to inform the queue that the events can be discarded (and to trigger any associated callbacks).

queue.Batch.ACK is used in the vicinity of actual ACK handling, which can make it confusing to follow the side effects of various similarly-named functions. Done more clearly separates ACK handling from general event cleanup.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-06-09T21:13:17.547+0000

• Duration: 75 min 26 sec

Test stats 🧪

Test	Results
Failed	0
Passed	22229
Skipped	1935
Total	24164

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)