Keep requests session open in S3Streamer

[rapto]

Description

When downloading many chunks of files, a supervisor thread is run for each of them, causing 'too many open files' error. This patch uses only one session per S3 instance.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.