[RFC] A iterator style poll CQ interface

[FujiZ]

Consider defining an iterator style poll CQ interface, which is more idiomatic and may be able to unify the old-style ibv_poll_cq interface with the new ibv_start_poll interface.

// create a iterator on cq; maybe we can pass some parameters to it
let mut iter = cq.poll();
// get one wc from the iterator
let wc = iter.next().unwrap();
// access the fields in wc through methods; 
let wr_id = wc.wr_id();
// for new-style poll interface, drop iterator will invoke `ibv_end_poll`
drop(iter);

[FujiZ]

We may need to return a reference to the WorkCompletion in this case, so https://stackoverflow.com/questions/68606470/how-to-return-a-reference-when-implementing-an-iterator might be helpful.

[dragonJACson]

9e0c959, for cq_ex, there is no real WorkCompletion object, all wr_id, opcode things are got from cq_ex or function calls.

That would be pretty different for basic cq.