release-20.2: colmem: limit batches of dynamic size by workmem in memory footprint

[yuzefovich]

Backport 1/1 commits from #59851.

/cc @cockroachdb/release

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This commit adds a minor improvement to ResetMaybeReallocate method so
that it reuses the old batch (without doubling the capacity and
allocating a new batch) when the old batch has enough capacity and
reached the provided limit in size. This change effectively allows us to
limit all batches flowing through the vectorized engine by workmem
setting in size since the main batch-producers (cfetcher and
columnarizer) use this method.

Addresses: https://github.com/cockroachlabs/support/issues/808.

Release note (sql change): Most batches of data flowing through the
vectorized execution engine will now be limited in size by
sql.distsql.temp_storage.workmem (64MiB by default) which should
improve the stability of CockroachDB clusters.

[cockroach-teamcity]

This change is 

[yuzefovich]

We probably will want to include 629a5f1 into this backport too.

[asubiotto]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)

[yuzefovich]

I think I'll briefly hold off on merging this for two reasons:

• so that the change bakes on master for a bit (the next 20.2 release starts on February 24th)
• and so that we can see which minor tweaks with regards to testing limits should be included.

[yuzefovich]

I haven't seen any fallout from recent work on making dynamic batch size based on memory usage, so I'll go ahead and merge it and will open up a few other backports with fixes too.