Deprecate the incremental mode of CMS

The incremental mode of CMS (i-CMS) was introduced when the train collector was removed in 2005/2006. This is a mode that targets single (or maybe dual) CPU machines. i-CMS is not used much and has been kind of made obsolete by the current trend towards multi core platforms.

The i-CMS code is expensive to maintain and add to the complexity in the GC code. We are currently testing i-CMS, but there are more important GC combinations to test. So, i-CMS uses up test resources that we could use for more widely used GC combinations.

To simplify the GC code and enable testing of all GC combinations that we support we should remove i-CMS. We can only remove a GC in an major release, so the plan is to remove it for the upcoming JDK8 release.