Multithreaded encryption and transfer

Hi! Thanks for the writeup - there’s a bunch of things in there that should be addressed, and some that are at the top of the todo list for 0.9 (i.e. the GUI does put too much CPU load on syncthing). I’m travelling at the moment with sporadic internet access and just about zero syncthing time, but I’ll get back to this during next week. :slight_smile: