The Photon completion ID used by the PWC API.
int photon_probe_completion(int proc, int *flag, int *remaining, photon_cid *comp, int *src, void(*cb)(photon_cid), int flags)
int photon_get_with_completion(int proc, uint64_t size, photonBuffer lbuf, photonBuffer rbuf, photon_cid local, photon_cid remote, int flags)
int photon_put_with_completion(int proc, uint64_t size, photonBuffer lbuf, photonBuffer rbuf, photon_cid local, photon_cid remote, int flags)