RFR: 8238634: Reduce log verbosity of the JFR thread sampler

[Erik Gahlin]

Hi,

Could I have a review of this fix.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8238634

Webrev:
http://cr.openjdk.java.net/~egahlin/8238634/

Testing:
tier1, tier2 + test/jdk/jdk/jfr

Thanks
Erik