ref https://linear.app/tryghost/issue/ENG-1518
After releasing the analytics job improvements, it appears for large
sites we're awfully close to missing some Mailgun events because of an
unexpected behavior of the aggregateStats call for just the opened
events job. This is taking 2-5x(+) the amount of time that the aggregate
queries take for the other jobs, despite not being dependent on the
events.
To err on the side of caution, we're going to roll this back and look to
optimize the aggregation queries before re-implementing. And we may be a
bit more cautious in giving _some_ but not _all_ priority to the
`opened` events.
ae15e12ffc