Add the ShutdownInitiated callback to Server.

The ShutdownInitiated callback is called at the beginning of the
shutdown process and is useful for logging the shutdown action.
Add NotifyClosed for signaling when hijacked connections are closed.

Hijacked http connection no longer notify the server of changes to the
connection via the ConnState callback. NotifyClosed will place the
connection in the remove channel so that it is recorded as closed.
Merge branch 'shutdown-callback'
Merge branch 'notify-closed'

Conflicts:
	graceful_test.go