[DNM] Aggregate syntax transform cleanup#25

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rafatower wants to merge 5 commits intoREL_11_CARTOfrom

aggregate-syntax-transform-cleanup

Commits on Sep 11, 2019

An experimental commit to try to push down aggregate functions
Rafa de la Torre
committed

Commits on Sep 16, 2019

Fix collation safety rules for aggregations
Rafa de la Torre
committed
Deparse Row Expression as subquery
Rafa de la Torre
committed
Add public to execution environment of postgres_fdw
Rafa de la Torre
committed

Commits on Sep 20, 2019

Fix collation in other instances (to be reviewed)
Rafa de la Torre
committed