Differentiate blocking unblocking collectives in UCC

[bosilca]

Provide a hint to UCC collective init to know it is dealing with a blocking collective.

[Sergei-Lebedev]

I think it would be easier to just modify COLL_UCC_REQ_INIT macro e.g.

#define COLL_UCC_REQ_INIT(_coll_req, _req, _coll, _module) do{          \
        if (_coll_req) {                                                \
            _coll.mask   |= UCC_COLL_ARGS_FIELD_CB;                     \
            _coll.cb.cb   = mca_coll_ucc_completion;                    \
            _coll.cb.data = (void*)_coll_req;                           \
        } else {                                                        \
            _coll.mask  |= UCC_COLL_ARGS_FIELD_FLAGS;                   \
            _coll.flags |= UCC_COLL_ARGS_HINT_OPTIMIZE_LATENCY;         \
        }                                                               \
        COLL_UCC_CHECK(ucc_collective_init(&_coll, _req,                \
                                           _module->ucc_team));         \
        if (_coll_req) {                                                \
            _coll_req->ucc_req = *(_req);                               \
        }                                                               \
    } while(0)

So you don't need to add extra arg and repeat same code in every collective

[janjust]

@bosilca when merged - can we get a v5.0.x cherry-pick?