Fix thread multiple performance hit

Pull over Jeff's original patch. Update it per George's comments: since opal_using_threads is always false if thread-multiple hasn't been enabled, there is no reason to include checks for it in that condition. So restore the #if statements in opal/threads/mutex.h to regain the remaining missing performance. Also, restore the thread debugging code as there is no reason to exclude it as it only builds in debug environments

No corresponding master commit as this is solely a 1.8 series issue

@jsquyres @bosilca @hjelmn  please review