…jects (aws#30209)

Fixes aws#30573.

I recently had the mispleasure of trying to empty a bucket with ~600000 objects using CDK's `autoDeleteObjects` feature. What I observed was that each lambda invocation would get through a few tens of thousands of objects in relatively good time (a few minutes), then the lambda would grind to a halt doing very little until it reached its 15 minute timeout. This process then repeats with subsequent invocations of the lambda.  I had to empty the bucket in the web console to make real progress toward deleting the bucket.

I have proven that the low memory allocated to the lambda (the default 128mb) plus this recursion is to blame. There is no need to recurse, and doing so will put pressure on the stack, the heap, and (because this is an async function) the event loop.

Switch the recursion to iteration.

aws#30209 (comment)

- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

…jects (aws#30209)

### Issue # (if applicable)
Fixes aws#30573.

### Reason for this change

I recently had the mispleasure of trying to empty a bucket with ~600000 objects using CDK's `autoDeleteObjects` feature. What I observed was that each lambda invocation would get through a few tens of thousands of objects in relatively good time (a few minutes), then the lambda would grind to a halt doing very little until it reached its 15 minute timeout. This process then repeats with subsequent invocations of the lambda.  I had to empty the bucket in the web console to make real progress toward deleting the bucket.

I have proven that the low memory allocated to the lambda (the default 128mb) plus this recursion is to blame. There is no need to recurse, and doing so will put pressure on the stack, the heap, and (because this is an async function) the event loop.

### Description of changes

Switch the recursion to iteration.

### Description of how you validated changes

aws#30209 (comment)

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*