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.
"180 - where it from?" here we calculate days which we should check in commits thread. If user's stats is empty - $last_scan will take PPGETSTAT_DEFAULT_TIME_PERIOD_FOR_SCANNING value that is ~180 days.