Shortcut when message can be sent immediately

[takluyver]

@asvetlov in #1 :

mandatory call_soon in .send_multipart() makes significant performance degradation -- usually ZMQ message may be sent immediately.

It would be fairly easy for send_multipart to check if there are no messages waiting, and if not, try to send one immediately, using the DONTWAIT flag, and then only queue it in the rare case that that's not possible.

The only reason I didn't do this is that I was writing the simplest possible thing I could use, and performance wasn't very important.

[asvetlov]

There is standard trade-off between simplicity and performance, the final choice is up to you.

But make a note if you'll implement shortcut eventually: after sending please check ZMQ socket for incoming messages -- you may never retrieve wakeup signal otherwise.

[takluyver]

Good point, thanks.