This removes the global logger (which by the way was deleted when the first tox
was killed, so other toxes would then stop logging). Various bits of the code
now carry a logger or pass it around. It's a bit less transparent now, but now
there is no need to have a global logger, and clients can decide what to log and
where.
An index for replacement candidate is searched
in one lookup cycle for all types (bad, possibly bad, good).
Sorting of items has been removed (sorting logic can be substituted
by a maximum search).