Preserve bridge download status across SETCONF, HUP

This code changes it so that we don't remove bridges immediately when
we start re-parsing our configuration.  Instead, we mark them all, and
remove all the marked ones after re-parsing our bridge lines.  As we
add a bridge, we see if it's already in the list.  If so, we just
unmark it.

This new behavior will lose the property we used to have that bridges
were in bridge_list in the same order in which they appeared in the
torrc.  I took a quick look through the code, and I'm pretty sure we
didn't actually depend on that anywhere.

This is for bug 3019; it's a fix on 0.2.0.3-alpha.