Proxy: send cancel notifications to all instances

[khanova]

Problem

If cancel request ends up on the wrong proxy instance, it doesn't take an effect.

Summary of changes

Send redis notifications to all proxy pods about the cancel request.

Related issue: #5839, https://github.com/neondatabase/cloud/issues/10262

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]

2436 tests run: 2318 passed, 0 failed, 118 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_crafted_wal_end[last_wal_record_crossing_segment]: debug

Postgres 14

• test_multixact: release

Code coverage (full report)

• functions: 55.9% (12836 of 22945 functions)
• lines: 82.5% (69218 of 83902 lines)

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_{The comment gets automatically updated with the latest test results
d913971 at 2024-02-13T11:36:52.862Z :recycle:}