[Refactor]: Change Validity to using "ASSERT" as default value and refactor the *_ms fields to using format_timestamp(validity)

[creatorrr]

📜 Description

Turns out cozo's "ASSERT" automatically stores timestamps with microseconds accuracy and so the *_ms fields that we use are not needed. https://docs.cozodb.org/en/latest/timetravel.html

👟 Relevant files

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