Use another last_checked timestamp for jobs

1) Add multiplier for core commits module to increase pages amount per day
2) Use last parsed commit/comment timestamp instead of week beginning timestamp for condition.
How many new pages for core commits created at drupal.org ?
If it is more than 6 we will never process all of them
cron task executed one time per minute, so we can easy spent ~1-2 hours per day on core commits parsing
Let's change this value to 120