Allow the pg layer to work for simple selects with the postgres_fdw

Another avenue for materialization support outlined in TEIID-4251 is to rely upon pg materialization, which would be easiest to setup through their foreign data wrapper. However it expects some additional support for:

start transaction syntax
abort transaction
automatic cleanup of portals/cursors at the end of a transaction
prepared cursors

Additionally this will have the same performance issue that we have with declare/fetch initially - that is a cursor requires a transaction, and our current strategy requires pre-buffering the results under a transaction. However since this would only be used for load/refresh that performance hit may be negligible - if now we'd need similar logic to effectively ignore the transaction wrapping.