Added index and modified datatype for sec and nsec to better reflex 32 bits content

[y-trudeau]

The index is needed for the purge-qan-data script. The sec and nsec column contain 32 bits unsigned values, why use 64 bits int?

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[delgod]

should we update old systems? or they can use old schema?

[askomorokhov]

Looks good to me 👍