Currently we have Threads::possibly_parallel_threads_do(), which operates on all the Java threads and the VM thread, but ignores other non-Java threads.  However, there are some places that want parallel iteration over all threads, which use explicit iteration and explicit use of the claim_threads_do mechanism.

It would be better to have a function similar to possibly_parallel_threads_do that operates on all threads.

There is a naming issue though.  Perhaps possibly_parallel_oops_do should be the all-threads variant, and the restricted version should be possibly_parallel_java_threads_and_vm_thread_do, being consistent with the naming of the serial iterators.