Change Server to contain a Vec of (ip, port) pairs to allow repeat listening

Expose Server::many for creating a Server that will listen on many (ip, port)
pairs.

Handler still receives a simple Iterator of (Request, Response) pairs.

This is a breaking change since it changes the representation of Listener,
but Handler and Server::http are unchanged in their API.

Fixes #7
Update client benchmarks for new Listening representation.