Make dispatch multithreading safe

Guard the execution of dispatch with a recursive thread lock.  This is
needed to make a thread safe way to send events to Client objects.  Note
that the only thread safe method is dispatch, everything else is unsafe
to call from another thread, as the thead handling the Client object
could be modifying arbitrary structures at any time.  In addition this
only keeps nasal demons away, and does not solve any of the difficult
syncronization issues that might be present.

discord/client.py

@@ -353,6 +353,7 @@ class Client(object):
     def __init__(self, **kwargs):
         self._is_logged_in = False
         self.connection = ConnectionState(self.dispatch, **kwargs)
+        self.dispatch_lock = threading.RLock()
         self.token = ''
         self.events = {
             'on_ready': _null_event,
@@ -435,18 +436,19 @@ class Client(object):
             object.__setattr__(self, name, value)
 
     def dispatch(self, event, *args, **kwargs):
-        log.debug("Dispatching event {}".format(event))
-        handle_method = '_'.join(('handle', event))
-        event_method = '_'.join(('on', event))
-        getattr(self, handle_method, _null_event)(*args, **kwargs)
-        try:
-            getattr(self, event_method, _null_event)(*args, **kwargs)
-        except Exception as e:
-            getattr(self, 'on_error')(event_method, *args, **kwargs)
-
-        # Compatibility shim to old event system.
-        if event_method in self.events:
-            self._invoke_event(event_method, *args, **kwargs)
+        with self.dispatch_lock:
+            log.debug("Dispatching event {}".format(event))
+            handle_method = '_'.join(('handle', event))
+            event_method = '_'.join(('on', event))
+            getattr(self, handle_method, _null_event)(*args, **kwargs)
+            try:
+                getattr(self, event_method, _null_event)(*args, **kwargs)
+            except Exception as e:
+                getattr(self, 'on_error')(event_method, *args, **kwargs)
+
+            # Compatibility shim to old event system.
+            if event_method in self.events:
+                self._invoke_event(event_method, *args, **kwargs)
 
     def _invoke_event(self, event_name, *args, **kwargs):
         try: