sql: use datum alloc for crdb_internal stmt stats rows

[dt]

Happened to observe a cluster running a customer test suite which included a query that inspected stmt stats often, causing the CRDB node to spend a considerable amount of CPU time in production of the stmt stats vtable, in particular allocating (and then GC'ing) individual datums, especially given how wide this table has become with the addition of storage stats.

This change uses a datum allocator to produce those rows to reduce the number of separate allocations from the runtime.

Release note: none.
Epic: 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 

[dt]

TFTR!

bors r+

[rafiss]

thanks for the find! i'm a +1 on backporting this if you are.

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[rafiss]

blathers backport 23.1 22.2

[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 86029ce to blathers/backport-release-22.2-105093: 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 failed. See errors above.

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_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}