Expect close in WsServer connection disconnect

Don't bother doing the prepareDisconnect dance if the connection is
already broken. This avoids having to wait until the connection times
out when a slave gives up on a current connection and starts a new one.

packages/master/src/BaseConnection.js

@@ -64,6 +64,7 @@ class BaseConnection extends libLink.Link {
 	}
 
 	async disconnect(code, reason) {
+		this.connector.setClosing();
 		try {
 			await libLink.messages.prepareDisconnect.send(this);
 		} catch (err) {