enforce ordering of libbeat API server Start/Stop functions

[leehinman]

Proposed commit message

Add strict ordering to the libbeat API server Start and Stop functions.

Without this it is possible to start the API server multiple times and for the calling of the Stop function to precede the calling of the Start function.

Not an issue in normal beats but in beats receivers when Start and Stop may be called quickly.

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.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

cd libbeat\api
go test . -v -count 1 -run TestOrdering

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @leehinman? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[AndersonQ]

given the failing tests pass, it's good

[elasticmachine]

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