RFR: Make adaptive default heuristics

[Roman Kennke]

Hi,

Since introducing degenerated marking and tweaking the heuristics a
little bit for the double-marking issue, adaptive heuristics have
become much better. They no longer tend to go into full-gc (I couldn't
push them into full-GC even under heavy load), and it makes better use
of heap headroom (dynamic tends to underuse the heap and leave ~20%
empty at the top).

I propose to make adaptive the default. In all benchmarks I tested it
with I've seen some increase in performance, or at least no regression
(doesn't make a difference on apps that are not GC intensive).

http://cr.openjdk.java.net/~rkennke/adaptive/webrev.00/

Ok?

Roman