RFR(M): 8245113: JFR Recorder Thread and JFR Periodic Task Thread to run in thread state "_thread_in_native"

[Markus Gronlund]

Greetings,

Please review this enhancement to let threads JFR Recorder Thread and JFR Periodic Task run in state _thread_in_native where possible.
We do this to reduce the amount of time safepoint synchronizations have to wait, unnecessarily, for these threads to reach a safepoint poll site.

Enhancement: https://bugs.openjdk.java.net/browse/JDK-8245113  
Webrev: http://cr.openjdk.java.net/~mgronlun/8245113/webrev/ 
Testing: jdk_jfr, stress testing

Thanks
Markus

PS. Includes some nits to fixup a previous changeset too (comments, spelling)