Intel compiler intrinsics for memory barriers
minor: update comments, add link
adjust LoadLoad macro for ARMv7 compatibility
reduce theoretical lock contention; defer cleanup

reduce theoretical lock contention by deferring map cleanup in
mcdb_mmap_thread_registration(), even though threaded programs
should prefer to register and hold onto the mcdb for multiple
queries, since the registration cost can be higher than the cost
of a single mcdb query.  (In the future, might store spinlock
in mcdb map to replace the current global mutex lock)
minor: header file rename (plasma_attr.h)
minor: header file rename (plasma_attr.h)