breaking change: tweaks to poll and PollItem
breaking change: tweaks to poll and PollItem

* PollItem should take lifetime to make sure it doesn't outlive a socket
* PollItem.revents should be private and turned into a method
* `poll` should return an `int` as it's the number of signaled events.

petehayes102/rust-zmq,tempbottle/rust-zmq,erickt/rust-zmq,cristicbz/rust-zmq,alexkamp/rust-zmq,pfernie/rust-zmq,dginev/rust-zmq,pblasucci/rust-zmq,jedisct1/rust-zmq