RFR: 8144996: Replace the HeapRegionSetCount class with an uint

[David Lindholm]

Hi,

Please review this cleanup patch that removes the HeapRegionSetCount 
class and replaces it with an uint.

Bug: https://bugs.openjdk.java.net/browse/JDK-8144996
Webrev: http://cr.openjdk.java.net/~david/JDK-8144996/webrev.00/

Testing: JPRT and running jmap executing the heapsummary command 
verifying that it still print correct values.


Thanks,
David