Possible thread safety issue with active_devices

[frankier]

The use of active_devices might not be thread safe if this library is used in more than one thread. Putting a new key in active_devices could cause it to be reallocated in one thread while another thread is reading which might cause undefined behaviour.

[Keno]

Yes, Julia didn't have threading when this package was written. We'll need to add some locks. That said, I'm not sure that the graphviz library itself is thread safe, so we'll want to investigate that first.