Handle credentials rotation

[emmaai]

Changes:

• removed the static credentials, let boto3.client handle the credentials rotation
• changed how s3_client (ref:https://github.com/opendatacube/odc-tools/pull/597) is managed on dask cluster, one for scheduler and one per work (among all threads rather than each per thread)

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (ec3cdd3) 76.38% compared to head (6e23284) 76.59%. Report is 2 commits behind head on develop.

Files	Patch %	Lines
odc/stats/io.py	54.54%	5 Missing :warning:
odc/stats/proc.py	55.55%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #114 +/- ## =========================================== + Coverage 76.38% 76.59% +0.21% =========================================== Files 34 34 Lines 3210 3205 -5 =========================================== + Hits 2452 2455 +3 + Misses 758 750 -8 ```

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