Added a short delay to `AccessTokenCache.getToken()` to avoid an async-busy-loop when sharing cache

[vcolin7]

Fixes: #30466, #28364.

Description

Added a short delay to AccessTokenCache.getToken() to avoid an async-busy-loop when the first thread to attempt the retrieval of a new token takes longer than usual and the cache is shared amongst many threads.

For more details follow this link.

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• [X] The pull request does not introduce [breaking changes]
• [X] CHANGELOG is updated for new features, bug fixes, or other significant changes.
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[azure-sdk]

API change check

API changes are not detected in this pull request.