fix(lambda): gate timeout spans and add missing clear

[duncanista]

What does this PR do?

Gates AWS Lambda tracer instrumentation with the key lambda. Adds a clearTimeout to non-promisified functions.

Motivation

Support ticket where customers was being timed out because they were using promises and returning the function as non-promisified, making it to successfully work with AWS Lambda, but not with our tracer instrumentation.

Plugin Checklist

• [x] Unit tests.
• [ ] TypeScript definitions.
• [ ] TypeScript tests.
• [ ] API documentation.
• [ ] CircleCI jobs/workflows.
• [ ] Plugin is exported.

Additional Notes

Disabled by DD_TRACE_DISABLED_INSTRUMENTATIONS=lambda,other,another by following the current workaround for instrumentations.