RFR/RFC: Passive should opt-in the barriers, not opt-out

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/passive-opt-in/webrev.01/

Current "passive" heuristics is diagnostic and only runs Full GCs. In that mode, barriers are not
required, and our Wiki mentions that is the way to dissect the barriers cost. As Shenandoah adoption
goes up, we might want to have this in more convenient form. That is, let's make barriers opt-in,
not opt-out with "passive" heuristics. This allows users to make diagnostic runs with all barriers
disabled at once, without knowing what barriers they need to disable. New code also prints the
barrier names it implicitly disables, so that users can selectively enable them back without looking
into the source or our wiki.

We still allocate fwdptr slot, because Full GC code needs it for storing relocation data.

Our existing testing covers all interesting tests with "passive" heuristics already.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey