Prevent deadlocks in scheduler shutdown (#268)

By splitting up the executor lock and jobstore lock, this allows jobs
that are shutting down to query their state from the database (requiring
`self._jobstore_lock`) without causing a deadlock.

Since setting `self.state` above prevents new jobs from being added to the
executors as they shut down, breaking up these locks won't allow new
jobs to run while the scheduler is shutting down.

apscheduler/schedulers/base.py

@@ -178,12 +178,13 @@ def shutdown(self, wait=True):
 
         self.state = STATE_STOPPED
 
-        with self._jobstores_lock, self._executors_lock:
-            # Shut down all executors
+        # Shut down all executors
+        with self._executors_lock:
             for executor in six.itervalues(self._executors):
                 executor.shutdown(wait)
 
-            # Shut down all job stores
+        # Shut down all job stores
+        with self._jobstores_lock:
             for jobstore in six.itervalues(self._jobstores):
                 jobstore.shutdown()