core (memory/sqla): generic `get` + `session_add` in `delete`: new generic `get` allow to retrieve an ORM object with given column and value, and optionally join load attributes. In `delete`, it is now possible to specify instances to add to session, this allow, to update them in the same transaction. It is also now possible to specify a list of object to delete them all in the same transaction. rel 367

this certificates are used to enable TLS during tests. Those must be the same as in
backend_e2e (they are generated there)