Heap verification in product builds

Allowing pre- and post-gc heap verification in product builds (and the ability
to switch the controlling boolean via jconsole) would help chase down
gc-related bugs in production deployments without the need to ship and
install an instrumented jvm. Fastdebug jvm's are notoriusly bad at
reproducing production errors, so having the capability in a product
build is desirable for easy reproduction and diagnosis of production
heap corruption errors.