Before it would prepend your requested entrynodes to your list of guard
nodes, but feel free to use others after that. Now it chooses only
from your EntryNodes if any of those are available, and only falls back
to others if a) they're all down and b) StrictNodes is not set.

Also, now we refresh your entry guards from EntryNode at each consensus
fetch (rather than just at startup and then they slowly rot as the
network changes).

The goal here is to make users less likely to set StrictNodes, since
it's doing closer to what they expect it should be doing.