[11u] RFR: 8258414: OldObjectSample events too expensive

[Florian David]

Hi,

Please review this 11u backport. It's very similar to the original patch,
except for the class loader data graph lock and JfrKlassUnloading::sort()
method which are not available in 11u.

The missing lock has been replaced by locking the module_lock instead,
as it is done in jfrcheckpointManager.cpp:363.
JfrKlassUnloading class does not exist and thus sort() method is not
replaced,
I added a TODO instead.

Bug: https://bugs.openjdk.java.net/browse/JDK-8258414
webrev: http://cr.openjdk.java.net/~fdavid/8258414/webrev.00/
Original PR: https://github.com/openjdk/jdk/pull/2780

Testing:
- Linux x86_64 tier1 tests
- SPECvm 2008 on a 96 cores/192 Gib server. JFR recording size is 75.12 MB
before the patch and 3.08 MB after the patch.

Let me know what you think.

Thanks,
Florian