Refactor the server module

[alexcrichton]

This commit refactors the server module as we discussed a bit offline.
The main changes are:

• The entry point is how Http, the implementation of ServerProto.
  This type has a new constructor as well as builder methods to
  configure it.

• A high-level entry point of Http::bind was added which returns a
  Server. Binding a protocol to a port requires a socket address
  (where to bind) as well as the instance of NewService. Internally
  this creates a core and a TCP listener.

• The returned Server has a few methods to learn about itself, e.g.
  local_addr and handle, but mainly has two methods: run and
  run_until.

• The Server::run entry point will execute a server infinitely, never
  having it exit.

• The Server::run_until method is intended as a graceful shutdown
  mechanism. When the provided future resolves the server stops
  accepting connections immediately and then waits for a fixed period of
  time for all active connections to get torn down, after which the
  whole server is torn down anyway.

• Finally a Http::bind_connection method exists as a low-level entry
  point to spawning a server connection. This is used by Server::run
  as is intended for external use in other event loops if necessary or
  otherwise low-level needs.

[seanmonstar]

Er, I didn't realize deleting that branch would close this. I had finished merging the tokio branch into master, and it was now out of date. Also, noticed there is a compile error in the benches/end_to_end.rs.

[alexcrichton]

Sure thing, I'll send the PR against master instead