If OMPI is initialized as thread multiple, then it is possible for
Portals events to be processed out of order by different threads.
Out of order events could lead to reactivation of the block
(PTL_EVENT_AUTO_FREE) before the block is removed from the active
list (PTL_EVENT_AUTO_UNLINK). This commit adds a status field to
ompi_mtl_portals4_recv_short_block_t that coordinates these events.