[8u] RFR: 8215727: Restore JFR thread sampler loop to old / previous behavior

[Ekaterina Vergizova]

Hello,
I would like to backport JFR issue 8215727 to 8u.

JBS: https://bugs.openjdk.java.net/browse/JDK-8215727
Original patch: https://hg.openjdk.java.net/jdk/jdk/rev/58154bf80f90
Webrev for 8u: https://cr.openjdk.java.net/~apetushkov/jfr_backports_katya/8215727/webrev.00/

The patch applies almost cleanly except for the third hunk of jfrThreadSampler.cpp.
It doesn't apply due to different context (iterating through java threads is different between 8u and 13), re-applied manually.

Tested with tier1 and jdk.jfr tests on Linux and Windows, no regression.

Thanks,
Ekaterina