Fix race condition in S3 Express identity cache

[jakob-keller]

Description of Change

A race condition in the S3 Express identity cache was introduced in 110b0a2a. This change modifies the cache to hold asyncio.Task instances instead of actual credentials to mitigate the issue without introducing locking.

Assumptions

Replace this text with any assumptions made (if any)

Checklist for All Submissions

• [x] I have added change info to CHANGES.rst
• [x] If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details): closes #1072
  • [ ] Detailed description of issue
  • [x] Alternative methods considered (if any): This race condition might also be resolved through locking. This would result in additional complexity and runtime overhead.
  • [x] How issue is being resolved: The cache holds asyncio.Task instances that wrap the underlying get_credentials coroutine.
  • [ ] How issue can be reproduced
• [ ] If this is providing a new feature (can be provided via link to issue with these details):
  • [ ] Detailed description of new feature
  • [ ] Why needed
  • [ ] Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• [x] I have read and followed CONTRIBUTING.rst
• [x] I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• [x] I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (110b0a2) 86.13% compared to head (9097884) 86.25%.

Files	Patch %	Lines
aiobotocore/utils.py	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1073 +/- ## ========================================== + Coverage 86.13% 86.25% +0.12% ========================================== Files 60 60 Lines 5863 5858 -5 ========================================== + Hits 5050 5053 +3 + Misses 813 805 -8 ``` | [Flag](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1073/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/aio-libs/aiobotocore/pull/1073/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs) | `86.25% <50.00%> (+0.12%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aio-libs#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jakob-keller]

@thehesiod: I'm looking forward to your review.

[thehesiod]

mind just bumping the release date then I'll merge and deploy. Thanks! very nice implementation