Bug 294348

We already have a lot of different profiling hooks that allow profiling logic to be injected into a lot of different code paths in libpas. However, it's hard to develop profiling policies that vary based on type of allocation or that are probabilistic. One nice mechanism for this is the local allocator, where at least for segregated allocations, we can toggle profiling for a specific local allocator based on some policy in the allocation medium-to-slow path, instead of burdening the fastest fast paths with additional logic. This can help us collect useful profiling information while reducing the impact on normal program performance characteristics.