Use NOWAIT when locking a table

lock is used to enforce a serializable level for all transaction using on the
same table. This is not achieved if transactions are waiting for the lock
instead of abort and retry with a new transaction because they will still have
the initial transaction row values with the repeatable read level.

issue4373
review5901002

trytond/backend/postgresql/database.py

@@ -356,7 +356,7 @@ class Cursor(CursorInterface):
         return self.cursor.fetchone()[0]
 
     def lock(self, table):
-        self.cursor.execute('LOCK "%s" IN EXCLUSIVE MODE' % table)
+        self.cursor.execute('LOCK "%s" IN EXCLUSIVE MODE NOWAIT' % table)
 
     def has_constraint(self):
         return True