Guard nodes not weighted by bandwidth

Tor clients are currently selecting guard nodes uniformly. The add_an_entry_guard call passes in a NULL state to choose_good_entry_server, which causes it to call router_choose_random_node with need_capacity=0.

I marked severity as High because this is obviously screwing with load balancing of the network and probably has a large negative impact on user experience. Also, a backport of the fix is essential since it really won't have any effect until most clients start using it.

[Automatically added by flyspray2trac: Operating System: All]