RFR: 8191564: Refactor GC related servicability code into GC specific subclasses

[Roman Kennke]

Currently, there's lots of GC specific code sprinkled over 
src/hotspot/share/services. This change introduces a GC interface for 
that, and moves all GC specific code to their respective 
src/hotspot/share/gc directory.

http://cr.openjdk.java.net/~rkennke/8191564/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/8191564/webrev.00/>

Testing: hotspot_gc and hotspot_serviceability, none showed regressions

Built minimal and server without regressions

What do you think?

Roman