Refactor smartlist_choose_node_by_bandwidth to be less horrible.

Refactor smartlist_choose_node_by_bandwidth to be less horrible.

With this patch, I dump the old kludge of using magic negative
numbers to indicate unknown bandwidths.  I also compute each node's
weighted bandwidth exactly once, rather than computing it once in
a loop to compute the total weighted bandwidth and a second time in
a loop to find which one we picked.