Reduce number of queries to DB
Make the out handler non-locking, queue will handle locking on the input, django/sql will handle locking on the output
QuerySet.exists() is slightly quicker

see https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.exists
Should probably use get(), we don't need to worry about an exception here
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