Fixes for the ShutdownInitiated callback.
Fixes for the ShutdownInitiated callback.

Call the ShutdownInitiated callback after the server has closed the
listener socket. Also removes the unecessary hook variable and switches
the callback's docs to a websocket example.

tylerb/graceful,no2key/graceful,LamCiuLoeng/graceful,anupcshan/graceful,francoishill/graceful,WatchBeam/graceful