This also replaces some int.to_bytes() calls with a faster
struct.pack (up to x4 faster). This approach is also around
x3 faster than creating a BinaryWriter just to serialize a
TLObject as bytes.
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.
