fix S3RemoteLogIO using incorrect transfer config

[stephen-bracken]

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The S3RemoteLogIO class was setting use_threads=False in the transfer config arguments, but this argument is ignored if using the (now default) CRTTransferManager instead of the S3TransferManager. Setting preferred_transfer_client="classic" will use the S3TransferManager instead.

https://docs.aws.amazon.com/boto3/latest/reference/customizations/s3.html#s3-transfers

use_threads – If True, threads will be used when performing S3 transfers. If False, no threads will be used in performing transfers; all logic will be run in the current thread. Note: This value is ignored when resolved transfer manager type is CRTTransferManager.

We noticed this because some of our tasks which had very large log files (a few GB each) were causing the worker pods in kubernetes to go OOMKilled when they were uploaded to S3.

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