Cherry-pick #24387 to 7.x: Fix failing installation on windows 7

[michalpristas]

Cherry-pick of PR #24387 to 7.x branch. Original message:

What does this PR do?

Issue described here: #24327
there was a race between enrollment process and restarting service, FS playing part as well.
The thing was that when agent was restarted only on windows 7 it loaded standalone ID, even though it was already replaced by enrollment process.
Then when agent retrieved hosts from fleet it even overwrote updated ID with stale one.

This fix adds a lock which prevents simultaneous write in between these two processes and a forced Reload in case of fleet managed agent later in the cycle.
Another thing is FSync after file rotation which was missing for windows.

These seems to fix the issue, tested on win 7 VM on cloud

Why is it important?

Fixes #24327

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.

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Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24409 opened

• Start Time: 2021-03-08T10:59:01.179+0000

• Duration: 55 min 0 sec

• Commit: 50a9f92

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Failed	0
Passed	45953
Skipped	4982
Total	50935

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Test	Results
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Passed	45953
Skipped	4982
Total	50935

[blakerouse]

Backport looks good.