RFR: 8210857: Allow retiring TLABs and collecting statistics in parallel

[Per Liden]

There is currently no good way of retiring TLABs and collecting their 
statistics in parallel. The current code for doing this also traverses 
the thread list twice, which is unnecessary. I propose that we adjust 
the TLAB API to allow this to be done in parallel, in a single pass.

A few notes on this patch:

* The GlobalTLABStats class is restructured into the 
ThreadLocalAllocStats class to allow for a more flexible way of 
collected statistics when retiring TLABs (flexible in the sense that it 
allows for parallel operations).

* The TLAB API is slightly adjusted. The "make_parsable(bool 
retire_tlab)" is broken into two functions, "make_parsable()" to just 
make a TLAB parsable, and "retire(ThreadLocalAllocStats* stats)" to 
retire and optionally collects statistics.

* GCs are unaffected by this change, in the sense that they continue to 
call CollectedHeap::ensure_parsability(). However, all GCs will benefit 
from now doing a single pass over the thread list instead of two to 
retire and collect stats.

* This is the last patch in my series of TLAB related patches. The 
remaining patches only touch ZGC to actually retire/resize/remap TLABs 
in parallel.


Bug: https://bugs.openjdk.java.net/browse/JDK-8210857
Webrev: http://cr.openjdk.java.net/~pliden/8210857/webrev.0

/Per