Support for multiple addresses per server

[gtaban]

From @helje5: https://github.com/swift-server/http/pull/81#issuecomment-342630664

An open question is, do we want to support multiple binds? I tend to say yes. This would modify the structure of the server a little.

(the other option is to use multiple servers if you want to listen on multiple addresses, but that would be kinda wasteful).

[rymcol]

+1 for multiple binds