fix: save execution stack in query as string

[durran]

Summary

While debugging some unexpected memory usage in a production cluster, we noticed that mongoose's Query object was constructing a new Error instance in order to be able to preserve the original stack when throwing an error after attempting to execute the query more than once. This is problematic in that until the stack is converted to a string, the error retains references to every object in the call site info. This PR changes the execution stack object to the actual stack string to avoid this problem. Since this variable appears intended to not be part of the public API it seems reasonable to change.

Examples

[durran]

Note see related more detailed reasoning for a similar change in Node itself: nodejs/node#34103 (comment)

[vkarpov15]

We also noticed some performance issues with the _executionStack pattern here. I think this PR confirms my suspicions that we should just remove _executionStack and originalStack in Mongoose 9.0 because those cause performance overhead on every single query, and the original rationale is somewhat obsolete. The original rationale was to help users debug duplicate queries, but the most common cause of duplicate queries was mixing await and callbacks, like await Model.find({}, function(err, docs) {}). Given that Mongoose no longer supports callbacks, duplicate queries are significantly less likely.

TLDR; thanks for the PR, LGTM. But we will probably remove the _executionStack logic in our next major release.