Fixing telescope push timestamp check

[alexcottner]

We updated the kinto client library and started using the changeset endpoint.
However that endpoint returns records sorted by bucket and collection name, causing this check to be a false positive.
Sorting the records by last_modified first makes this check work and prevents us from hitting this banana peel in the future.

[leplatrem]

Good catch. This regression was introduced in #1515

The fix would work, but I'd like to contrast it with other solutions:

• Switch back to get_records() in the check code since it's what's used in the lambda https://github.com/mozilla-services/remote-settings-lambdas/blob/9e32cf6d9a4d6f9dec5d3d583c3f973c3184a51a/commands/sync_megaphone.py#L53-L62

• Use the changeset endpoint in the sync megaphone job too Use changeset endpoint for sync_megaphone job remote-settings-lambdas#1506 Since it's the endpoint that is used by clients to poll for changes.

• Sort entries explicitly by -last_modified in the changeset endpoint, since there's no reason the list not to be sorted. And align both records and changeset Sort entries by -last_modified in monitor/changes changeset endpoint mozilla/remote-settings#725

I approve the PR anyway in case we want the check to be green before our holidays :)

[alexcottner]

I vote we make this green before the holidays and then follow up.

If we modify how changeset sorts things, there might be a few other impacts to think about. Like the remote-settings-devtools users. But that would be a clean solution if we make everything use changeset and it sorts things in an expected way.