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/util.py

@@ -372,6 +372,7 @@ class DaemonThread(threading.Thread, Logger):
         self.jobs = []
         self.stopped_event = threading.Event()        # set when fully stopped
         self.stopped_event_async = asyncio.Event()    # set when fully stopped
+        self.wake_up_event = threading.Event()  # for perf optimisation of polling in run()
 
     def add_jobs(self, jobs):
         with self.job_lock:
@@ -405,6 +406,8 @@ class DaemonThread(threading.Thread, Logger):
     def stop(self):
         with self.running_lock:
             self.running = False
+            self.wake_up_event.set()
+            self.wake_up_event.clear()
 
     def on_stop(self):
         if 'ANDROID_DATA' in os.environ: