RFR: 8209976: Improve iteration over non-JavaThreads

[Kim Barrett]

> On Aug 27, 2018, at 12:09 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> Hm, except that I seem to have forgotten to remove the
> now obsolete asserts from GlobalCounter::critical_section_begin/end.

New webrevs:
full: http://cr.openjdk.java.net/~kbarrett/8209976/open.01/
incr: http://cr.openjdk.java.net/~kbarrett/8209976/open.01.inc/