RFR (S) 8246274: G1 old gen allocation tracking is not in a separate class

[Luo, Ziyi]

Hi,

Could you please review this change which refactors 
G1Policy::_bytes_allocated_in_old_since_last_gc into a dedicated new 
tracking class G1OldGenAllocationTracker?

Bug ID:
https://bugs.openjdk.java.net/browse/JDK-8246274
Webrev:
http://cr.openjdk.java.net/~phh/8246274/webrev.00/

Testing: Local run hotspot:tier1.

This is the first step toward improving the G1 old gen allocation tracking. As 
described in JDK-8245511, we will further add humongous allocation tracking 
and refactor G1IHOPControl::update_allocation_info(). This is a clean 
refactoring of the original G1Policy::_bytes_allocated_in_old_since_last_gc 
field and G1Policy::add_bytes_allocated_in_old_since_last_gc() method.

Thanks,
Ziyi