By setting printing to false and not restarting the sender thread when the print_thread exits in the disconnect method this should allow printcore to disconnect even if the print thread is running.

In particular this makes it possible to ctrl+c out of prontserve durring an m109 where it previously errored out and hung with the following error:

Can't read from printer (disconnected?) (SerialException): device reports readiness to read but returned no data (device disconnected?)