ERP5Catalog assumes that dates in catalog are UTC, this works
fine with DATETIME columns that does not carry any timezone
information, but TIMESTAMP columns are converted to mariadb timezone,
which is by default system timezone.

If we want ERP5Catalog to query on TIMESTAMP columns with the existing
logic of converting dates to UTC (because "dates are in UTC in
catalog"), we need to have TIMESTAMP columns in UTC.

This also generally seem a better default than falling back to system
timezone.

/reviewed-on nexedi/slapos!623