Plugins: make Plugins.stop() faster

Plugins.stop() is now part of the normal shutdown flow (since 90f39bce88),
so this shaves up to 100 ms off that (avg: 50 ms).
It is also waited on in around ~60 unit tests: this saves 3 sec.

electrum/plugin.py

@@ -209,7 +209,7 @@ class Plugins(DaemonThread):
 
     def run(self):
         while self.is_running():
-            time.sleep(0.1)
+            self.wake_up_event.wait(0.1)  # time.sleep(0.1) OR event
             self.run_jobs()
         self.on_stop()