Continue polling clients in priority queue if the top is not ready

[michael-disalvo]

The fair queue iterator implementation returns Poll::Pending when there may be other streams in the priority queue that are ready. Instead of returning after checking only the top stream, continue checking until a stream that returns Poll::Ready is found, or all streams have returned Poll::Pending and will resubmit themselves to the priority queue when they are ready.

I had an issue that can be summarized as the following: I had a server with a pull socket, and two clients connecting with push sockets. One client was fast and sent a message every second. The other client was slow and sent a message every 5 seconds. I expected the server to receiver 5 messages from the fast client, and then a single message from the slow client, and that this would repeat. I observed the eventually, the server would receive only single message from the fast client until a single message from the slow client was received. I debugged this and realized that when the server's fair-queue contained both clients, only the slow client would get polled, leaving the fast client in the queue, even though it was ready.

To reproduce this, I created a demo repo at https://github.com/michael-disalvo/zeromq-demo. Clone it, and run 3 processes:

• cargo run --bin server
• cargo run --bin client -- -s 1 -m fast
• cargo run --bin client -- -s 5 -m slow

The server will print out when it receives each message. At first, every second a message will be received from the fast client. Eventually, it will get stuck and the server will receiver a message from the fast client only every 5 seconds.