RFR: 8307348 - Parallelize heap walk for ObjectCount(AfterGC) JFR event collection [v7]

[olivergillespie]

> ObjectCount(AfterGC) event does a full single-threaded heap scan at a safepoint. After https://bugs.openjdk.org/browse/JDK-8215624, it is trivial to use the parallel version of the heap scan, reducing the time spent at the safepoint, and thus reducing the overhead of this event.
> 
> The performance improvement is obvious, but just for confirmation, on my 16-core host, at around 1GB occupancy:
> 
> 
> Before: 770ms ( [3.059s][debug][gc,phases         ] GC(13) Report Object Count 770.317ms )
>  After:  92ms ( [2.335s][debug][gc,phases         ] GC(13) Report Object Count  91.742ms )
> 
> 
> Question 1: Should this be the default behaviour for populate_table (use the number active workers as the parallelism, if nothing else specified)?
> 
> Question 2: Is active_workers the correct value to use here? Or is max_workers more appropriate?

olivergillespie has updated the pull request incrementally with one additional commit since the last revision:

  Fix use of WithActiveWorkers
  
  My scope was incorrect, thanks @albertnetymk
  Also fix ResourceMark usage, thanks Aleksey.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13774/files
  - new: https://git.openjdk.org/jdk/pull/13774/files/b6da2f73..ac1d234e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13774&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13774&range=05-06

  Stats: 14 lines in 2 files changed: 6 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13774.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13774/head:pull/13774

PR: https://git.openjdk.org/jdk/pull/13774