[FIX] base_action_rule: do not repeatedly trigger already-executed rules based on time condition

A programming error introduced at rco@openerp.com-20121220142445-emzzvhlw400q37c9
fails to properly check when a rule has already been executed in the past.
Time-based rules should only be executed if the trigger date is in the past and:
 - either they never executed before
 - or the last execution date is older than the trigger date