datastore: fix two inserts in tx state with unique idx

[Centril]

Description of Changes

Fixes https://github.com/clockworklabs/SpacetimeDB/issues/2601.

This fixes two bugs, where if you:

1. define a table with struct T { #[unique] x: u32 }
2. insert T { x: 42 } twice => bug 1, will panic but shouldn't.
3. find row for 42 => empty iterator => bug 2

API and ABI breaking changes

There was a bug where inserting twice caused a unique constraint violation, but again, it's a bug.

Expected complexity level and risk

4 - the datastore is getting messy.

Testing

• The test test_set_semantics was buggy and is now fixed.
• A regression test add_twice_and_find_issue_2601 is added for bug 2.