[multi-sources] support for moving an entity from an external source (closes #343818) Original need is to move a user from a ldap source to the system source so we can delete it from ldap without loosing information into the cubicweb instance. We can't wait for the user to be deleted from the ldap since it will be too late then to get back user attributes, so it has to be a manual operation to operate before actual deletion. This makes sense for other sources as well. So the idea is to make the "Any cw_source CWSource" relation editable by managers, and to watch changes of it. We then check the move is possible (ie from an external source to the system source) and do necessary stuff (essentially changing source information and copying data into the system source). Remaining pb is that we don't want the moved entity to be reimported later. To distinguish this state, the trick is to change the associated record in the 'entities' system table with eid=-eid while leaving other fields unchanged, and to add a new record with eid=eid, source='system'. External source will then have consider case where `extid2eid` return a negative eid as 'this entity was known but has been moved, ignore it'. Notice no ui is provided yet, it has currently to be done in a c-c shell.

[repository api] definitly kill usage of word 'pool' to refer to connections set used by a session Also document session's data storage and some other internals. Hopefuly things will get clearer. Closes #1684860: vocabulary confusion in repository code: notion of 'pool'