AMLII-1842 Fix a potential deadlock on fork

[vickenty]

What does this PR do?

Fix a potential deadlock on fork

Description of the Change

If a fork happens while some thread is sending metrics and holds one of the locks, the lock would remain locked in the child process and would deadlock, either in the post_fork handler (when the post_fork hook tries to close the socket), or later (when user code tries to send a metric).

Work around the issue by resetting the socket and buffer locks in the child process. If those were locked in the parent at the time of the fork, the internal client state may be inconsistent, so we reset it as well.

With config lock, we can not reset the state to some known good state, and to avoid problems when fork is called while a thread modifies the client configuration, the config lock will be held across fork. Both the client and the parent can safely unlock it afterwards.

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

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