[server] improve TZDatetime support
by depending on logilab-database 1.15.0 including
https://www.logilab.org/ticket/1485893, we can now keep the tzinfo
attribute on datetime objects for TZDatetime attributes, so one knows
that this is a tz-aware datetime.
To easily make it work with backends that have no tz support, we keep
converting tz-aware datetime objects into utc naive datetime objects
before sending them to the database.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')