Postgres knows whether a given aggregate is parallel-safe, and creates
parallel aggregation plans based on that. The `partialize_agg` is a
wrapper we use to perform partial aggregation on data nodes. It is a
pure function that produces serialized aggregation state as a result.
Being pure, it doesn't influence parallel safety. This means we don't
need to mark it parallel-unsafe to artificially disable the parallel
plans for partial aggregation. They will be chosen as usual based on
the parallel-safety of the underlying aggregate function.
From PG12 on, CREATE OR REPLACE is supported for aggregates,
therefore, since we have dropped support for PG11, we can avoid
going through the rigamarole of having our aggregates in a separate
file from the functions we define to support them. Nor do we need to
handle aggregates separately from other functions as their creation
is now idempotent.
