RFR: 8027643 - Merge GenCollectorPolicy and TwoGenerationCollectorPolicy

[Jesper Wilhelmsson]

Hi,

This is another step in cleaning up the collector policy code. It is the first 
of a few related patches. This change merges the GenCollectorPolicy class with 
TwoGenerationalCollectorPolicy.

I have deliberately refrained from doing any other cleanups that is made 
possible by this change. That will come in later changes. That makes this change 
really easy to review :-)

Webrev: http://cr.openjdk.java.net/~jwilhelm/8027643/webrev/

Bug: https://bugs.openjdk.java.net/browse/JDK-8027643

Thanks!
/Jesper