Bug 1020290 - factor UpdateJitActivations.  r=shu
Bug 1020290 - remove inert code.  r=shu
Bug 1020290 - move notions of 'nursery' out of ThreadSafeContext, rename 'fjNursery'.  r=shu
Bug 1020290 - reduce the proliferation of names by introducing overloading.  r=shu