RFR: 8212656: Clean up heap filler insertion

[Per Liden]

While reviewing JDK-8211955, I suggested that the filler insertion code 
in CollectedHeap should be broken out into a Fill class (to match the 
somewhat related Copy class we already have). The filler insertion code 
doesn't have to live in the already crowded CollectedHeap class, as it's 
basically just a bunch of utility functions (just like the functions in 
the Copy class).

With this patch CollectedHeap calls Fill::initialize() in it's 
constructor. This will set up the default behavior. Collectors with 
special needs (G1, ZGC and Shenandoah) can then call one or more of the 
Fill::set_*() functions to adjust filling behavior.

Bug: https://bugs.openjdk.java.net/browse/JDK-8212656
Webrev: http://cr.openjdk.java.net/~pliden/8212656/webrev.0

Testing: Passed tier{3,5,6} on all Oracle supported platforms, running 
additional tiers right now.

/Per