fix(schema) declare timestamps as number to keep millisecond precision

[kikito]

Currently the created_at the field in the new DAO uses second-level numbers instead of millisecond-level numbers (since we use ngx.time instead of ngx.now)

This changes that and makes all timestamps be stored with millisecond precision (no changes to the database are required, just that we replaced ngx.time by ngx.now).

However this is a breaking change: until now the created_at field looked like an integer, and now it will start looking like a float.

This change is required by #3622

[Tieske]

My guess would be that this is because of the time order required by the targets?

An alternative would be to replace the UUID by a ULID. In a ULID, part of the randomness is sacrificed and replaced with a time stamp. The way this is done ensures that you can lexically sort based on the ULID, and get historical order.

A ULID is compatible (as a format) with UUID

But I don't know what the impact of that would be regarding breaking changes.