Weasel says circuit_get_by_conn is his main timesink.  Most of its
users were just checking whether OR conns had circuits, so add a
circuit count to OR conns, and check that. One was
circuit_about_to_close_conn, which was doing an O(n^2) series of calls
to get all circs on an OR conn, so make an O(n) function for that.
Finally, circuit_get_by_edge_conn was using it as a sanity test that
has been around for a while but never found any actualy insanity, so
kill that.

circuit_get_by_conn is finally dead, which is good, since it was never
sane to begin with.


svn:r5460