RFR: 8077836: Make sure G1ParGCAllocBuffer are marked as retired

[Stefan Johansson]

Hi,

Please review this fix for:
https://bugs.openjdk.java.net/browse/JDK-8077836

Webrev:
http://cr.openjdk.java.net/~sjohanss/8077836/hotspot.00/

Summary:
At the end of a G1 gc a call to G1ParGCAllocator::retire_alloc_buffers 
is made, in this code we should make sure all alloc buffers are retired. 
This is currently handle by calling flush_and_retire_stats for each 
active buffer. Because this method is only implemented for 
ParGCAllocBuffer and not for G1ParGCAllocBuffer we currently miss 
updating the _retired field in G1ParGCAllocBuffer.

The _retired field is checked in a guarantee in the G1ParGCAllocBuffer 
destructor, but this destructor is not usually called because we are 
missing a virtual destructor for G1ParGCAllocator.

This patch adds this destructor and implements 
G1ParGCAllocBuffer::flush_and_retire_stats by calling the version in 
ParGCAllocBuffer and setting the _retired field to true.

Testing:
Manual verification that G1 always fails with the new destructor and 
that the fix avoids this.

Thanks,
Stefan