RFR: Break heuristics out from ShCollectorPolicy into their own source files

[Roman Kennke]

Another one that I wanted to do since a long time, and which now is
needed in preparation for the interleaving of minor/major GC change.

- Break out all the heuristics into their own source files
- split declarations from definitions into .hpp/.cpp files


This was a pretty mechanic refactoring, no logic changes (hopefully).

http://cr.openjdk.java.net/~rkennke/shheuristics/webrev.00/

Tested: building release/fastdebug PCH/no-PCH, hotspot_gc_shenandoah

Ok?