Reduce number of database connection creations during PG tests

I put in a quick System.out.println() statement into the constructor of PostgresConnection, and about 8! times a connection is created up to the first assertion in RecordsStreamProducerIT.shouldReceiveChangesForInsertsWithDifferentDataTypes(). This should be limited as much as possible, e.g. by keeping open the connection used on the test side of things.