Request for review: 6976350 G1: deal with fragmentation while copying objects during GC

[Tao Mao]

6976350 G1: deal with fragmentation while copying objects during GC
https://jbs.oracle.com/bugs/browse/JDK-6976350

webrev:
http://cr.openjdk.java.net/~tamao/6976350/webrev.00/

changeset:
Basically, we want to reuse more of par-allocation buffers instead of 
retiring it immediately when it encounters an object larger than its 
remaining part.

(1) instead of previously using one allocation buffer per GC purpose, we 
use N(=2) buffers per GC purpose and modify the corresponding code. The 
changeset would easily scale up to whatever N (though Tony Printezis 
suggests 2, or 3 may be good enough)

*(2) Two places of cleanup: allocate_during_gc_slow() is removed due to 
its never being called.
                                               access modifier (public) 
before trim_queue() is redundant.