Fix thread safety race condition on _running flag

[enaboapps]

Summary

• Protect _running flag with lock in _start_http_transport, _schedule_flush, disconnect, and disconnect_async
• Prevents race condition between auto-flush timer thread and disconnect

Closes #9

Test plan

• All 52 tests pass

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/thread-safety-running-9

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