This once maybe made sense when ExitNodes meant "Here are 3 exits;
use them all", but now it more typically means "Here are 3
countries; exit from there." Using non-Fast/Stable exits created a
potential partitioning opportunity and an annoying stability
problem.
(Don't worry about the case where all of our ExitNodes are non-Fast
or non-Stable: we handle that later in the function by retrying with
need_capacity and need_uptime set to 0.)
