postgres_fdw: Use psql variables for connection parameters

postgres_fdw: Use psql variables for connection parameters

Several statements need to reference the current connection's current
database name and current port value.  Until now, this has been
accomplished by creating dynamic SQL statements inside of a DO block,
which is not as easy to parse.  It also takes away some of the
granularity of any error messages that might occur, making debugging
harder.

By capturing the connection-specific settings into psql variables, it
becomes possible to write simpler SQL statements for the FDW objects.
This eliminates most of DO blocks used in this test, making it a bit
more readable and shorter.

Author: Author: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/CADkLM=cpUiJ3QF7aUthTvaVMmgQcm7QqZBRMDLhBRTR+gJX-Og@mail.gmail.com