Return a server future so users can decide how to run their server

[shaladdle]

cc @tikue

Currently, we force users to call server.listen(reactor_handle, addr);. Forcing users to use a handle is a little opinionated, since users of the futures API might want to be able to explicitly control how they run their server on their reactors.

This would change FutureService to expose a bind call, which would either return the bound address and a future which encapsulates the server itself, or just a future which exposes a local_addr method. Then users can choose how to run this future on their reactor(s).

[tikue]

Closed by #199. The users now have control over how they run their server future.