kv: remove dependency on ticks from maybeDropMsgApp

[nvanbenschoten]

Related to #73838.

In d77bee9, we stopped ticking uninitialized replicas, so we can no longer use ticks as a proxy for the age of a replica in the escape hatch of maybeDropMsgApp. Instead, we now use the age of the replica directly. We hit the escape hatch for any replica that is older than 20s, which corresponds to the 100 ticks we used before.

[cockroach-teamcity]

This change is 

[tbg]

Reviewed 2 of 2 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @nvanbenschoten)

[nvanbenschoten]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

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

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error creating merge commit from 63dfc14 to blathers/backport-release-21.1-74108: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

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

Backport to branch 21.1.x failed. See errors above.

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

Backport to branch 21.2.x failed. See errors above.

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