RFR/ITM: Britney - rewrite logging

[Niels Thykier <niels@thykier.net>]

Hi,

I have written a branch that replaces most of our ad-hoc log framework
with the standard python3 logging framework.  The branch is available from:

https://anonscm.debian.org/cgit/users/nthykier/britney.git/log/?h=rewrite-logging

The only notable difference is that the timestamp format changes from
"Sat Mar 24 04:01:19 2018" to "2018-03-24T09:40:09+0000".  This can
trivially be changed if another format is desired.

Original format:
> I: [Sat Mar 24 04:10:15 2018] - > Update complete - Verifying non-installability counters

New format:
> I: [2018-03-24T09:40:09+0000] - > Update complete - Verifying non-installability counters

Notable omission: The solver's debug log has not been converted yet, but
that can be done after the merge.

Thanks,
~Niels