RFR (M) 8210155: Lock ClassLoaderDataGraph

[coleen.phillimore at oracle.com]

Summary: In preparation for concurrent class unloading.

open webrev at http://cr.openjdk.java.net/~coleenp/8210155.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8210155

Tested with mach5 hs-tier1-7, which tests JFR and JVMTI changes. Also 
tested with performance dev-submit testing with no regression.

Thanks,
Coleen