Join Push down - If we have a join between two foreign tables from the same remote server, push that join down to the remote server instead of fetching all the rows for both the tables and performing a join locally.

Fixes crash with IMPORT FOREIGN SCHEMA when executed repeatedly.
Restricts fetching of the system attributes from the remote relation.
Fixes compiler warning due to clash with MySQL library macro.
Costs are not shown in the explain output when the COSTS explain option is OFF.
Uses environment variables in tests for connection parameters.
No longer supports PG/EPAS version 9.5.