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.
BREAKING CHANGE: `Server` is no longer the pimary entry point. Instead,
an `Http` type is created and then either `bind` to receiver a `Server`,
or it can be passed to other Tokio things.
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