review request (S+) - 6962947 taskqueue statistics

[John Coomes]

Reviewers for my changes to taskqueue overflow handling (6957084) were
unanimous in suggesting that the existing taskqueue-related statistics
in PSPromotionManager should be more widely available.  This fix
cleans them up, moves them to TaskQueue and enables them by default in
debug builds.

http://cr.openjdk.java.net/~jcoomes/6962947-taskqueue-stats

-John