make sure to send down timestamp queries to vttablet

[systay]

Description

The time zone system setting was not being fetched correctly, which lead to it being ignored when evaluating now().

Related Issue(s)

Fixes #16820

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
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• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required

[vitess-bot[bot]]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 69.50%. Comparing base (538dd4c) to head (ca83068). Report is 93 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vcursor_impl.go	83.33%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16819 +/- ## ========================================== + Coverage 68.98% 69.50% +0.52% ========================================== Files 1562 1568 +6 Lines 200690 202480 +1790 ========================================== + Hits 138449 140743 +2294 + Misses 62241 61737 -504 ```

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[systay]

Is it not incorrect to route these queries to a random shard? If someone runs:

START TRANSACTION;
SELECT * FROM `users` WHERE `id` = 1;
SELECT NOW();
COMMIT;

Where the users table is sharded on id, one might expect that the SELECT NOW() is run on the same machine as the SELECT FROM users, so that the transaction stays single-shard and so that NOW() returns the time at the start of the transaction, which is how unsharded MySQL works.

Good point. I'll update the PR and try again.

[systay]

Replaced by #16824