Make confirm_received a flag, avoid race conditions, fix bg RPCError

There was a race condition between TelegramBareClient.invoke
receiving part and MtProtoSender._handle_rpc_result actually
reading the result after setting request.confirmed = True.

The .confirmed is now a threading.Event to avoid the sleep(0.1).

RPC errors have been moved inside the request so they're not
raised on a background thread anymore.

telethon/telegram_bare_client.py

@@ -294,8 +294,9 @@ class TelegramBareClient:
 
         try:
             self._sender.send(request)
-            while not request.confirm_received:
-                sleep(0.1)  # TODO Use a proper lock
+            request.confirm_received.wait()  # TODO Optional timeout here?
+            if request.rpc_error:
+                raise request.rpc_error
             return request.result
 
         except ConnectionResetError: