This new filter mode removes Tor circuits that don't match the
provided fingerprints and leaves TGen transfers/streams untouched. At
the same time the visualize mode only includes TGen transfers/streams
with an existing mapping between TGen transfers/streams and Tor
streams/circuits.

This patch changes the default behavior of the visualize mode. The
original behavior of visualizing TGen transfers/streams *without* an
existing mapping to Tor streams/circuits can be selected with the
--outer-join switch, even though that's rather an edge use case.

Another minor change is that the filtered analysis files is not
written with sort_keys=True anymore, which would have produced a newly
sorted file with keys in alphabetic order rather than the original
insert order. The result is an actually useful diff.