Move unreachable port warnings to a periodic event.

[nmathewson]

Arguably, the conditions under which these events happen should be a
bit different, but the rules are complex enough here that I've tried
to have this commit be pure refactoring.

Closes ticket 25952.

Finally, before this code goes away, take a moment to look at the
amazing way that we used to try to have an event happen
every N seconds:

  get_uptime() / N != (get_uptime()+seconds_elapsed) / N

Truly, it is a thing of wonder. I'm glad we didn't start using this
pattern everywhere else.

[coveralls]

Coverage decreased (-0.004%) to 58.874% when pulling b396e4e on nmathewson:ticket25952 into 60a2c92 on torproject:master.