Now RPC results can be received from the updates thread, as long
as they are errors. This, however, should not happen!
A recursive lock is now used (and released on every method, rather
than only on the `.receive()` one)
The .empty() function returns an "empty" instance of the object
(attributes set to None). This is used rather than using reflection.
The msgs_ack handling broke stuff (InvokeWithLayer when there were
updates), so this is now handled; yet there may be a better fix
