Proposed fix for Issue #1811.

1. Adds @synchronized decorator to provide thread-based locking support
2. Added @synchronized around common.generate_ssl_cert

• Add tests for any new features and bug fixes. Ideally, each PR should increase the test coverage.
• Follow the existing code style (e.g., indents). A PEP8 code linting target is included in the Makefile.
• Put a reasonable amount of comments into the code.
• Separate unrelated changes into multiple pull requests.
• 1 commit per PR: Please squash/rebase multiple commits into one single commit (to keep the history clean).