Discard circuit paths on which nobody supports ntor

Right now this accounts for about 1% of circuits over all, but if you
pick a guard that's running 0.2.3, it will be about 6% of the circuits
running through that guard.

Making sure that every circuit has at least one ntor link means that
we're getting plausibly good forward secrecy on every circuit.

This implements ticket 9777,
Excise the insertion_time_elem_t logic

It's now redundant with the inserted_time field in packed_cell_t

Fixes bug 10870.
Merge remote-tracking branch 'public/feature9777_024_squashed' into maint-0.2.4
Fix geoip by falling back to registered countries.

See 1d2179bc900f1646a5491b65294e78b175e70056 in master for details.

"""
Fall back to registered country if necessary.

When extracting geoip and geoip6 files from MaxMind's GeoLite2 Country
database, we only look at country->iso_code which is the two-character ISO
3166-1 country code of the country where MaxMind believes the end user is
located.

But if MaxMind thinks a range belongs to anonymous proxies, they don't put
anything there.  Hence, we omit those ranges and resolve them all to '??'.
That's not what we want.

What we should do is first try country->iso_code, and if there's no such
key, try registered_country->iso_code which is the country in which the
ISP has registered the IP address.

In short: let's fill all A1 entries with what ARIN et. al think.
"""
Merge remote-tracking branch 'origin/maint-0.2.3' into maint-0.2.4
Merge remote-tracking branch 'public/no_itime_queue' into maint-0.2.4