RFR(s): 8077415: Remove duplicate variables holding the CollectedHeap

[Per Liden]

Hi,

Universe::_collectedHeap points to the current CollectedHeap. However, 
collectors also have their own static instances for no real good reason. 
This patch removes all but the Universe::_collectedHeap and let users 
call Universe::heap() where needed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077415

Webrev: http://cr.openjdk.java.net/~pliden/8077415/webrev.0/

(This patch build on top of 
http://cr.openjdk.java.net/~pliden/8077413/webrev.0/ which is currently 
out for review)

cheers,
/Per