'last seen' data may be hours out of date

[richvdh]

We try to lock the whole user_ips table to do updates. This has two bad effects:

• postgres keeps trying to start autovacuums which it then has to abort
• when postgres finally gives up and does the autovacuum anyway, no user_ips updates happen until the vacuum completes, which can take several hours.

[richvdh]

search keywords: locked upsert

[richvdh]

fixing this would nomally be done by adding a unique constraint to the table so that we can do an upsert without locking the whole table; however this is complicated by the fact that the table currently has a bunch of duplicate entries.

[richvdh]

(note also that the dups are still happening so it's not as simple as clearing out the dups before adding the constraint)

[richvdh]

should be fixed by #4306