feat(server): add Server::with_graceful_shutdown method

This adds a "combinator" method to `Server`, which accepts a user's
future to "select" on. All connections received by the `Server` will
be tracked, and if the user's future finishes, graceful shutdown will
begin.

- The listener will be closed immediately.
- The currently active connections will all be notified to start a
  graceful shutdown. For HTTP/1, that means finishing the existing
  response and using `connection: clone`. For HTTP/2, the graceful
  `GOAWAY` process is started.
- Once all active connections have terminated, the graceful future
  will return.

Closes #1575

src/common/mod.rs

@@ -1,4 +1,5 @@
 mod buf;
+pub(crate) mod drain;
 mod exec;
 pub(crate) mod io;
 mod lazy;