[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__]

Move entity cache from web.request.ConnectionCubicWebRequestBase to repoapi.ClientConnection ResultSet._build_entity relies on an entity cache to avoid going round in circles (stack overflow due to infinite recursion) e.g. in a postcreate script. Clear the cache at commit/rollback time to avoid unexpected side effects; and explicitly clear the cache in a few tests where we mix ORM and RQL using the same connection.