Use an asyncio.Event instead of an asyncio.Lock for global rate limits.

There were some dead-locking issues that I suspect were due to the
way the global rate limit was handled. This changes it into a simple
Event that allows multiple coroutines to pass through instead of one
by one.

discord/http.py

@@ -92,7 +92,8 @@ class HTTPClient:
         self.connector = connector
         self._session = aiohttp.ClientSession(connector=connector, loop=self.loop)
         self._locks = weakref.WeakValueDictionary()
-        self._global_lock = asyncio.Lock(loop=self.loop)
+        self._global_over = asyncio.Event(loop=self.loop)
+        self._global_over.set()
         self.token = None
         self.bot_token = False
 
@@ -126,9 +127,9 @@ class HTTPClient:
 
         kwargs['headers'] = headers
 
-        if self._global_lock.locked():
+        if not self._global_over.is_set():
             # wait until the global lock is complete
-            yield from self._global_lock
+            yield from self._global_over.wait()
 
         yield from lock
         with MaybeUnlock(lock) as maybe_lock:
@@ -172,15 +173,16 @@ class HTTPClient:
                         is_global = data.get('global', False)
                         if is_global:
                             log.info('Global rate limit has been hit. Retrying in {:.2} seconds.'.format(retry_after))
-                            # acquire the global lock and block all processing
-                            yield from self._global_lock
+                            self._global_over.clear()
 
                         yield from asyncio.sleep(retry_after, loop=self.loop)
+                        log.debug('Done sleeping for the rate limit. Retrying...')
 
                         # release the global lock now that the
                         # global rate limit has passed
                         if is_global:
-                            self._global_lock.release()
+                            self._global_over.set()
+                            log.debug('Global rate limit is now over.')
 
                         continue