[atlas] Measurement result issues for some RIPE Atlas anchors

In April, we started investigating a strange traceroute result reported
by one of the RIPE Atlas anchors. The conclusion of our investigation is
that the probe upgrade process for the v3 anchor firmware has been
leaving old measurement daemons running as well as starting new ones.

This means that a gradually increasing number of measurements from the
affected anchors have been performed multiple times more or less
simultaneously. Because those processes were writing to the same file,
this resulted in a lot of garbled results. Purely by chance, the weird
result that triggered the investigation had the first part of one result
and the last part of another result in a way that generated
syntactically correct JSON.

The net result is that for the last few months, measurements done by
anchors may have resulted in multiple results at roughly the same time
or no results because the results were rejected as invalid. For those
who wish to check whether results for specific measurements were
affected, the following links will direct you to lists of probe IDs for
the affected anchors:

https://atlas.ripe.net/api/v2/probes/?is_anchor=true&tags=system-virtual&fields=id
(VM anchors)

https://atlas.ripe.net/api/v2/probes/?is_anchor=true&tags=system-v3-pc-engines&fields=id
(v3 hardware anchors)

We are working to resolve the issue and will keep you updated as and
when we have more to report. We apologise for any inconvenience this
might cause.

Philip