RFR (M) 8201655: Add thread-enabled support for the Heap Sampling

[JC Beyler]

Hi all,

When working on the heap sampling, I had promised to do the per thread
event so here it is!

Could I get a review for this:
Webrev: http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.02/
Bug: https://bugs.openjdk.java.net/browse/JDK-8201655

I was thinking of adding GC-dev for the memAllocator change once I get
favorable reviews for the rest of the change.

I've done a bit of performance testing and on the Dacapo benchmark I see no
change in performance when turned off (logical, any code change is behind a
flag check already in place) and when turned on it is comparable to the
current performance.

(More information is: I see a very slight degradation if we are doing 512k
sampling but no degradation at 2MB).

Thanks,
Jc
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