spiderpig: Store timestamp in DB as floats

Use time.time() to construct timestamps to store in the spiderpig
database. This is better for SQLite which would otherwise store
datetimes as local time strings.

This change requires deleting any existing spiderpig database. If
you're using local-dev, you can use local-dev/clean-apiserver to
delete old state.

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Related MRs:

• !657 (merged) (This MR)
• !656 (merged)
• master