allstacks: improve buffer alloc sizing and adopt more widely

[tbg]

In an escalation^1, we observed that on larger clusters, multiple rounds of runtime.Stacks are required until the buffer sizing algorithm as produced a large enough buffer. Since grabbing stacks will cause stop-the-world pauses, this is best avoided.

NB: *serverpb.StacksRequest is already populated using allstacks.Get.

Epic: none

[cockroach-teamcity]

This change is 

[tbg]

TFTR!

bors r+

[craig[bot]]

Build succeeded:

• local_roachtest_fips
• check_generated_code
• examples_orms
• linux_amd64_build
• local_roachtest
• linux_amd64_fips_build
• lint
• unit_tests
• acceptance
• docker_image_amd64

[blathers-crl[bot]]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 480248d3b3385fd4d6c8da27cdb5d1e4f744b94d to blathers/backport-release-24.3-159279: POST https://api.github.com/repos/tbg/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch release-24.3 failed. See errors above.

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