kv: expose env var to configure raft entry cache size

[nvanbenschoten]

Informs #98666.

This commit introduces a new COCKROACH_RAFT_ENTRY_CACHE_SIZE which can be used to configure the size of the raft entry cache. The default value is 16 MiB.

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[erikgrinaker]

Worth a backport?

[nvanbenschoten]

bors r=erikgrinaker

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

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 c309edc to blathers/backport-release-22.2-105799: 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 22.2.x failed. See errors above.

---

error creating merge commit from c309edc to blathers/backport-release-23.1-105799: 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 23.1.x failed. See errors above.

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