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
168c7d2155