transient failures in aws-s3 tvm from sts timeouts

# Bug
`2021-06-07T10:25:43.454Z [@adobe/aio-tvm] error: aws call timed out after 20000, are credentials set?`
https://github.com/adobe/aio-tvm/blob/master/lib/impl/AwsS3Tvm.js#L49

# Possible causes to investigate
- If this happens on high load or on many requests for the same namespace: there might be a limit in the number of sts credentials

Start with lib/impl/AwsS3Tvm.js at line 49 and trace the AWS STS call that reports the 20-second timeout. Investigate whether the failure correlates with high load or repeated requests for one namespace, as suggested in the issue. Done means the timeout cause is isolated and a verified fix or clear follow-up is identified.

Written by the indexing model from the issue text.