Store last run and duration of cron tasks

Users have no way to see when a scheduled task was run for the last time. Neither how long does it take to run.

This is particularly useful for long running task like the web_shop_product_data_feed for example.

So I propose to store the when a task started and ended in a ir.cron.log table. Also as this is constantly growing table, a task will clean it after some time (like for the ir.error table).