Refactor Poller class

A small refactoring of the Poller class.

I split up the #each method by taking the `poll = ZMQ::Poller.new` assignment and moving it into a memoized method, and I also moved the `for socket in sockets` loop into a method that is called on initialize.

This has the benefit of not having to new up a `ZMQ::Poller` object and register all sockets any time #each is called on the Flatware `Poller` class.