[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__]
if versions_map['cubicweb'][0] == (3, 7, 4):
config['http-session-time'] *= 60
config['cleanup-session-time'] *= 60
config['cleanup-anonymous-session-time'] *= 60