The `cagg_watermark` function perform just read-only operations so is
safe to make it parallel safe to take advantage of the Postgres
parallel query.
Since 2.7 when we introduced the new Continuous Aggregate format we
don't use partials anymore and those aggregate functions
`partialize_agg` and `finalize_agg` are not parallel safe, so make no
sense don't take advantage of Postgres parallel query for realtime
Continuous Aggregates.
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