Cleanup to statistics:

Cleanup to statistics:
* Removed 'pcache_miss_invalid' from stats.php and clear_stats.php, no longer used
* Added missing 'job-insert' and 'job-pop' to clear_stats.php
* Added missing call to wfIncrStats( 'pcache_miss_absent' ) when there's no key in ParserCacge::get()
* Removed useless 'pcache_not_possible' stat from OutputPage::addWikiTextTitle() since that function is mostly used for interface messages