Xpra: Ticket #1583: hello startup-complete race

If a window is being resized (ie: another client keeps it mapped, or using shadow / desktop servers and randr) the new client may get a window-move-resize packet before it has had a chance to initialize the packet handlers, causing a warning to be printed.

Looks like we need an internal "startup-complete" ("handshake"? "hello"?) state flag to ignore all such events until we've been able to send the hello back. (which sets the authenticated packet handlers) Alternatively, the client could null route them.

Similar problem for clipboard packets, which we may process before authentication is complete..