RFR: 8210713: Let CollectedHeap::ensure_parsability() take care of TLAB statistics gathering

[Per Liden]

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.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210713
Webrev: http://cr.openjdk.java.net/~pliden/8210713/webrev.0

/Per