Fix fs.walkStream to properly end async iterators

[gpittarelli]

What is the purpose of this pull request?

Attempting to use walkStream's returned stream.Readable as an async iterator leads to loops that never complete:

for await (const entry of walkStream(".")) {}
console.log("Never happens");

What changes did you make? (Give an overview)

Fixed StreamProvider's custom destroy() implementation to use the callback parameter which is required to correctly finish the stream.

Added a 'should support async iteration' test which demonstrates the issue on master, but works with this change.

[mrmlnc]

Thank you for the contribution!

I apologize for the delay in reviewing and approving your pull request. I am planning to release a new version this week.