[IMP] mail: avoid sending an email twice

In some specific conditions (e.g. admin logging during cron run, concurrent update), writing on the mail object may fail, triggering a rollback of the transaction.
As the write was done after the SMTP sendmail action, an email could be sent twice (state not correctly put, would be retried at next process of the email queue).
This patch provoke the error before the sendmail action and thus avoids sending the email.
Fixes #552
[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 which is less likely to break another transaction.
Fixes #552