Stop devicelist when client is stopped

To avoid the devicelist trying to save after the client has been
stopped

Hopefully will fix random test failures on node 11.

src/crypto/index.js

@@ -221,6 +221,7 @@ Crypto.prototype.start = function() {
 /** Stop background processes related to crypto */
 Crypto.prototype.stop = function() {
     this._outgoingRoomKeyRequestManager.stop();
+    this._deviceList.stop();
 };
 
 /**