Improve Pathfinder server responsiveness during massive client disconnect

[m-ronnblom]

In a situation where a large number of clients are connected to a pafd instance, and they all disconnect at the same time, the Pathfinder server becomes unresponsive until it has finished processing the disconnecting clients. Besides simply cleaning up resources used (memory, file descriptors etc), the server also need to mark services as orphans, unpublish services and process subscriptions matching those newly-unpublished services as well.

For example, with 10k clients, it will take on the order of 10 s for pafd to process them.

If possible, pafd should prefer processing of still-connected clients over disconnected ones.