RFR(S) 8227755: Need more than 2 distinct CodeCache unloading cycles

[Zhengyu Gu]

Shenandoah may run out of memory during concurrent nmethod evacuation 
and upgrade to full GC. Therefore, it needs at least 3 distinct 
CodeCache unloading cycle numbers.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227755
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227755/webrev.00/

Test:
   hotspot_gc (fastdebug and release) on Linux x86_64
   Submit tests.

Thanks,

-Zhengyu