fix: harmonydb: Use timestampz instead of timestamp across the schema

[magik6k]

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Proposed Changes

It appears that pgx doesn't pay attention to time.Time timezone field, and fails to convert the time value to whatever Timezone the backing postgres (yb) is configured to run; At the same time our schema uses the timestamp [without time zone] type all over the place, and that type accounts for time is units of seconds starting at postgres epoch as defined in the databases local timezone.

To make postgres store time in terms of seconds since postgres epoch in UTC we need to switch to timestampz (iiuc same type as TIMESTAMP WITH TIME ZONE), or alternatively do SET TIME ZONE 'UTC' on all connections - tho that option is much less robust imo (falls apart when external application connects to the database which will then think that it is in a different TZ).

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