Juju cannot upgrade to 1.26-alpha1

Well, status is nice and clear now:

machines:
  "0":
    agent-state: error
    agent-state-info: 'upgrade to 1.26-alpha1 failed (giving up): move lastlogin and
      last connection to their own collections: E11000 duplicate key error index:
      juju.userLastLogin.$_id_ dup key: { : "admin" }'
    agent-version: 1.26-alpha1

Puts blame pretty clearly on the commit that changed things.

Reverted the breaking change:

<https://github.com/juju/juju/pull/3145>

It can be relanded with upgrade fix next week.

I was able to easily reproduce this by running "watch juju status" during an upgrade to 1.26.

The problem stems from users accessing the API during the upgrade. Running status during the upgrade results in the login time being recorded for the user before the MigrateLastLoginAndLastConnection step runs. That step will then fail because it tries to insert a doc for that user's last login time, but the doc already exists.

We could fix this by inserting the lastlogin docs individually and ignoring duplicate key errors. We shouldn't use Upsert in this case, as what's written to the new lastLoginDoc will be more up to date than what's read in through the usersC collection.

Juju-CI verified that this issue is Fix Released in juju-core master:
    http://reports.vapour.ws/releases/3019