Tie PollItem's lifetime to its originating Socket's
Tie PollItem's lifetime to its originating Socket's

Also remove a couple unused lifetime parameters.

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