RFR(S): 8215893: Add better abstraction for pinning G1 concurrent marking bitmaps.

[Kharbas, Kishor]

Hi,
When I worked on JDK-8211425<https://bugs.openjdk.java.net/browse/JDK-8211425>, there was a request for better abstraction for pinning G1's CM bitmaps. RFE for the request is here - JDK-8215893<https://bugs.openjdk.java.net/browse/JDK-8215893>.

Here is a proposal : http://cr.openjdk.java.net/~kkharbas/8215893/webrev.00/

Here G1PageBasedVirtualSpace pins the entire reserved memory to memory during construction. The constructor takes an additional bool flag which says "does it need to pin the memory".
If the memory is pinned, '_special' flag is set to true. I piggy back on _special flag's behavior which is to not do actual OS (un-)commits on calls to (un)commit().
Rest of the changes is the mechanism to pass this flag from CM bitmaps creation in G1CollectedHeap all the way to G1PageBasedVirtualSpace.

Let me know if this is a good abstraction and if there is any better way.

Thanks
Kishor