Tor has a feature to preserve unrecognized state file entries in
order to maintain forward compatibility.  But this feature, along
with some unused code that we never actually removed, led to us
keeping items that were of no use to the user, other than at worst
to preserve ancient information about them.

This commit adds a feature to remove obsolete entries when we load
the file.

Closes ticket 40137.

Nick Mathewson authored Sep 23, 2020 and David Goulet committed Oct 07, 2020

It's way too easy to have a warning in the middle of the configure
process that the user never notices.

David Goulet authored Jul 01, 2020 and Nick Mathewson committed Oct 07, 2020

If at least one service is configured as a version 2, a log warning is emitted
once and only once.

Closes #40003

Signed-off-by: David Goulet <dgoulet@torproject.org>