[svn-r6919] completion counter cleanup (adds MPID_cc_t)
When compiled for fine-grained threading, the completion counter serves as a form of lockfree signalling. As such, atomic access and memory barriers must be used to ensure correctness. In per-object mode, this code also contains valgrind client request annotations to inform Helgrind/DRD/TSan about the lockfree signalling pattern. No reviewer.