[IMP] base: safer locking at user login

When a users connects, a lock is taken on the res_user table to modify the last login date. If another running transaction uses a foreign key to res.users (e.g. write_uid column), postgres may detect the update as a concurrent update and rollback the transaction.
In pg 9.3, the lock_strength parameter 'NO KEY' allows a weaker lock is less likely to break another transaction.