RFR (S) 8232112: MDO extra_data_lock leaks during class unloading

[coleen.phillimore at oracle.com]

Summary: call the MDO destructor during class unloading.

Also moved the other C heap deallocation from method_data to 
release_C_heap_structures.  Methods won't have method_data unless 
they're deallocated from the InstanceKlass.  The other paths to 
deallocate_contents are before the method is called.

Tested with tier1 all Oracle platforms, and tier2-6 on linux-x64.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8232112.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8232112

Thanks,
Coleen