pagectl: separate xy margin for draw timeline

[skyzh]

Problem

We were computing margin by lsn range, but this will cause problems for layer maps with large overlapping LSN range. Now we compute x, y margin separately to avoid this issue.

Summary of changes

before:

we have a lot of rectangles of negative width, and they disappear in the layer map.

after:

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]

1016 tests run: 973 passed, 0 failed, 43 skipped (full report)

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Flaky tests (1)

Postgres 15

• test_remote_storage_upload_queue_retries[local_fs]: release

_{The comment gets automatically updated with the latest test results
ae38aae at 2023-07-07T16:21:13.879Z :recycle:}

[bojanserafimov]

I'm not very proud of this code 😅 Thanks for the fix