fix(baseapp): select txs correctly with no-op mempool (backport #17769)

[mergify]

This is an automatic backport of pull request #17769 done by Mergify.
Cherry-pick of 63d046d has failed:

On branch mergify/bp/release/v0.47.x/pr-17769
Your branch is up to date with 'origin/release/v0.47.x'.

You are currently cherry-picking commit 63d046dd5.
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	both modified:   CHANGELOG.md
	deleted by us:   baseapp/abci_utils.go
	deleted by us:   baseapp/abci_utils_test.go
	both modified:   baseapp/baseapp.go

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

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[alexanderbez]

Changes committed:

• Move util stuff to abci_utils.go
• Undo breaking changes from original PR
• Update ProcessProposal handler
• Move test to baseapp_test.go

@julienrbrt would appreciate a quick glance that I didn't mess anything up.

[julienrbrt]

utACK