Release the lock as soon as possible when computing the snapashot for CQ visibility

[mimowo]

This is a follow up for review remark: https://github.com/kubernetes-sigs/kueue/pull/1069/files/3b1f7c19d16b67a267d30ba8945ae1a3bb2b501e#r1319099583

What would you like to be cleaned:

The sort operation can be done once the lock is released.

Why is this needed:

Improve performance of Kueue by reducing lock contention.

[tenzen-y]

/assign @lowang-bh

[k8s-ci-robot]

@tenzen-y: GitHub didn't allow me to assign the following users: lowang-bh.

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In response to this:

/assign @lowang-bh

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