RFR (M): 8233919: Incrementally calculate the occupied cards in a heap region remembered set

[Thomas Schatzl]

Hi all,

   may I have reviews for this change that ultimately makes sure that 
the number of occupied cards in a remembered set is only growing by 
providing a per-OtherRegionsTable count that is atomically updated when 
adding a remembered set entry.

Note that this count may not be completely accurate due to races when 
deleting a PerRegionTable (which is a known issue) from an 
OtherRegionsTable; but that is no different than before.

This helps improving the predictions in the young gen remset sampling 
thread, and increase the performance of getting the occupancy count.

Based on JDK-8233997, and JDK-8233998 also out for review.

CR:
https://bugs.openjdk.java.net/browse/JDK-8233919
Webrev:
http://cr.openjdk.java.net/~tschatzl/8233919/webrev/
Testing:
hs-tier1-5,

Thanks,
   Thomas