v2.1: simplify forward packet batches (backport of #3642)

[mergify]

Problem

logic in this structure can be simplified

Summary of Changes

• Use NonZeroU64 to avoid doing checked_div every packet
• Remove branching
• make testing simpler

Fixes #

---

This is an automatic backport of pull request #3642 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2cb11a7 has failed:

On branch mergify/bp/v2.1/pr-3642
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit 2cb11a7e2c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/banking_stage/forward_packet_batches_by_accounts.rs

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[bw-solana]

LGTM