RFR(s): 8077417: Cleanup of Universe::initialize_heap()

[Per Liden]

Hi,

Universe::initialize_heap() is kind of messy as it contains lots of #if 
INCLUDE_ALL_GCS, this patch tries to make that code a bit more readable. 
Also, all collectors except ParallelScavenge take their CollectorPolicy 
as an argument to the constructor. This patch aligns ParallelScavenge to 
this pattern to further make the code in initialize_heap() more readable.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077417

Webrev: http://cr.openjdk.java.net/~pliden/8077417/webrev.0/


This patch build on top of two other patches currently out for review, 
these are:
   http://cr.openjdk.java.net/~pliden/8077413/webrev.0/
   http://cr.openjdk.java.net/~pliden/8077415/webrev.0/

cheers,
/Per