[source/native] refactor eid generation code
This set of related methods clutters the native source API.
It will be better served with a dedicated eid genrator object.
Related to #3526594.
[jcr: allocate the eid generator in __init__]
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')