Consider using `debug_assert` in `MemTable::new`

[Centril]

See https://github.com/clockworklabs/SpacetimeDB/pull/869/files#r1497853181.

[Manojseetaram]

I went looking to pick this up, but MemTable::new no longer exists in
the codebase — searched crates/ and there's no MemTable type at all
anymore (likely refactored away since this issue was filed in Feb 2024).
Looks like this issue may be stale. Should it be closed?