RFR(S) 8038405: Clean up some virtual fucntions in Space class hierarchy

[Mikael Gerdin]

Hi,

I'm doing some cleanups in the "Space" classes to simplify the code for the G1 
class unloading work[1].
This change moves around some virtual functions in Space to more (in my mind) 
sane places:

* Space::is_in is implemented the same by all Space subclasses, move it to 
Space and make it non-virtual.
* Space::used_region has a bogus implementation that is never useful, remove 
it.
* Consolidate more code related to saved_marks in the Space base class
* Make CompactibleSpace::reset_after_compaction a pure virtual.
* Give Space::minimum_free_block_size a default implementation since only CMS 
needs to override it. 

Bug: https://bugs.openjdk.java.net/browse/JDK-8038405
Webrev: http://cr.openjdk.java.net/~mgerdin/8038405/webrev.0

/Mikael

[1] http://openjdk.java.net/jeps/156