Travis failures with Postgres tests

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Description

Travis builds are failing semi randomly now that we're testing against Postgres. Tests pass when rerun manually. I suspect a database contention or cleaning issue, but I haven't reproduced it reliably yet.

Inizia esaminando la configurazione di Travis e la configurazione dei test Postgres, quindi esegui nuovamente la suite che non passa per confrontare gli errori intermittenti con le riesecuzioni manuali. Analizza la contesa del database o il comportamento di pulizia segnalati; il lavoro è completato quando i test Postgres passano costantemente in Travis.