FINALFUNC_MODIFY set to shareable?

[davidkohn88]

Several aggregates should probably have the FINALFUNC_MODIFY flag (https://www.postgresql.org/docs/12/sql-createaggregate.html#SQL-CREATEAGGREGATE-NOTES) set to SHAREABLE in the create aggregate call...this is mostly to prevent them being used in window functions when they will have potentially odd behaviors...I think we should talk about a) whether we need to do this and b) which functions we need to do it for.