RFR: 8266723: JFR periodic events are causing extra allocations

[Jaroslav Bachorík]

Please, review the following fix.

JIRA:  https://bugs.openjdk.java.net/browse/JDK-8266723
Webrev:
 - hotspot: http://cr.openjdk.java.net/~jbachorik/8266723/hotspot/00/webrev/
 - jdk: http://cr.openjdk.java.net/~jbachorik/8266723/jdk/00/webrev/

In short, the JFR backport for JDK 8 could not bring in the whole JVM
logging framework and took a several shortcuts. Unfortunately, one of
the shortcuts is causing the log messages always generated but not
persisted resulting in possible disturbance in GC due to excessive
allocations.

The gist of the patch is binding the java side JFR logger log level
check with the global 'LogJFR' flag which is already used in the
native JFR logger implementation.

The path builds fine and passes tier1 and jfr tests on linux. Since
the changes are not platform specific I am expecting the tests passing
on other platforms as well.
If there is a volunteer able to run tests for this patch on more
platforms it would be welcome.

Cheers,

-JB-