Provide a way to shutdown a connection on quiescence

[carllerche]

Both the server and the client need a way to initiate a clean shutdown. The process should be:

• Stop accepting new streams
• Wait until all existing streams are complete
• Send a GO_AWAY frame
• Close the connection.

[teohhanhui]

Isn't this already supported? #245 #250

This issue is still referenced from the docs:

h2/src/server.rs

Lines 57 to 58 in 45b9bcc

	//! Graceful shutdown of the server is [not yet
	//! implemented](https://github.com/hyperium/h2/issues/69).

[jorendorff]

This issue is still referenced from the docs. @seanmonstar Is any part of this still open or did you fix all this 5+ years ago?