Fix legacy issue with context.done callbacks

[DarcyRaynerDD]

What does this PR do?

Fixes issue where lambda wrapper was never completes, due to an undocumented/deprecated API for completing the invocation. See here

Motivation

Because the wrapper was never completing, but the invocation was, trace spans would end up joined together in a mega trace.

Testing Guidelines

I've added unit tests, and also tested this manually to verify the fix.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)