proxy: reuse sess_id as request_id for the cplane requests

[khanova]

Problem

https://github.com/neondatabase/cloud/issues/11599

Summary of changes

Reuse the same sess_id for requests within the one session.

TODO: get rid of session_id in query params.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2718 tests run: 2582 passed, 0 failed, 136 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 28.1% (6286 of 22345 functions)
• lines: 47.0% (44213 of 94093 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
ca06e60 at 2024-03-26T11:40:52.273Z :recycle:}