sql/stats: forecast for different columnsets at different times

[michae2]

Before this change, all statistics forecasts for a table were at the same future time, determined as:

  time of most recent statistics collection (for any columnset)
  + average time between automatic collections (incl. all columnsets)

This commit changes the formula slightly to:

  time of most recent statistics collection (for **this** columnset)
  + average time between automatic collections (incl. all columnsets)

Meaning columnsets that were not included in the most recent statistics collection will now have an older forecast time than columnsets that were included. Columnsets that were included in the most recent collection will still all have the same forecast time.

This will have two effects on the optimizer:

1. When using the first table statistic to get a row estimate for the table, statistics builder will now favor forecasts of columnsets included in the most recent statistics collection over forecasts of columnsets not included.
2. Forecasts of columnsets not included in the most recent statistics collection will be now be more similar to their most recent collection, but also potentially more stale.

Fixes: #103958

Release note (bug fix): Fix a rare bug where stale multi-column table statistics could cause table statistics forecasts to be inaccurate, leading to unoptimal query plans.

[cockroach-teamcity]

This change is 

[mgartner]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @cucaroach, @michae2, and @msirek)

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-- commits line 23 at r1:
nit: included in the ...

[msirek]

Nice fix!

Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @cucaroach and @michae2)

[michae2]

Thanks for the reviews!

bors r=mgartner,msirek

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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