[c++] Improve default buffer sizes

[johnkerl]

Issue and/or context: As on #2682: switch the default from small to medium. The small/CI regime is preserved via opt-in environment variable.

See also https://github.com/single-cell-data/TileDB-SOMA/pull/531 wherein I found that even 512MB was too big for Windows CI -- which is why on this PR I have explicit CI logic.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.19%. Comparing base (c6bea4d) to head (819a982).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2681 +/- ## ======================================= Coverage 90.19% 90.19% ======================================= Files 37 37 Lines 4019 4019 ======================================= Hits 3625 3625 Misses 394 394 ``` | [Flag](https://app.codecov.io/gh/single-cell-data/TileDB-SOMA/pull/2681/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data) | Coverage Δ | | |---|---|---| | [python](https://app.codecov.io/gh/single-cell-data/TileDB-SOMA/pull/2681/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data) | `90.19% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data#carryforward-flags-in-the-pull-request-comment) to find out more. | [Components](https://app.codecov.io/gh/single-cell-data/TileDB-SOMA/pull/2681/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data) | Coverage Δ | | |---|---|---| | [python_api](https://app.codecov.io/gh/single-cell-data/TileDB-SOMA/pull/2681/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data) | `90.19% <ø> (ø)` | | | [libtiledbsoma](https://app.codecov.io/gh/single-cell-data/TileDB-SOMA/pull/2681/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=single-cell-data) | `∅ <ø> (∅)` | |

[eddelbuettel]

Thanks for making the change. I think you still need to alter the default in

https://github.com/single-cell-data/TileDB-SOMA/blob/c6bea4d2fd4f08849c992a70597e4b30dd568d56/libtiledbsoma/src/soma/column_buffer.h#L55