Use BTreeMap to store live timers

[faysou]

Pull Request

NautilusTrader prioritizes correctness and reliability, please follow existing patterns for validation and testing.

• I have reviewed the CONTRIBUTING.md and followed the established practices

Summary

• In order to have parity with what is used for TestClock and have some control over which timer is executed first if they fire at the same time. This can be useful for example to ensure that a spread aggregator fires before a bar aggregator, based on timer names.

Related Issues/PRs

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Improvement (non-breaking)
• Breaking change (impacts existing behavior)
• Documentation update
• Maintenance / chore

Breaking change details (if applicable)

Documentation

• Documentation changes follow the style guide (docs/developer_guide/docs.md)

Release notes

• I added a concise entry to RELEASES.md that follows the existing conventions (when applicable)

Testing

Ensure new or changed logic is covered by tests.

• Affected code paths are already covered by the test suite
• I added/updated tests to cover new or changed logic

[cjdsellers]

Thanks @faysou

This is a good refinement and improves parity between the clock types 🙏