RFR(M) 8224115: Shenandoah: Eliminate RWLock that protects recorded nmethod data array

[Zhengyu Gu]

I would like to eliminate RWLock that used to protect recorded nmethod 
data array in ShenandoahCodeRoots, because the methods that update the 
array, are already guarded by CodeCache_lock.

The patch also helps concurrent class unloading work, by avoiding two 
lock dances, which are deadlock prone.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224115
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224115/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu