Let CollectedHeap::ensure_parsability() take care of TLAB statistics gathering

A call to CollectedHeap::ensure_parsability(retire_tlabs), where retire_tlabs is true must always be preceded by a call to CollectedHeap::accumulate_statistics_all_tlabs(), otherwise we will loose TLAB statistics as it will be reset when the TLABs are retired. Instead of having all collectors do this dance and risk getting it wrong, I propose that we move this responsibility into ensure_parsability(), which will gather the TLAB statistics if it's told to retire all TLABs.