Re-implement PostgresStorageNode to work like RocksStoragenode

[linas]

The Postgres backend is old and crufty and slow and is missing modern features (e.g. frame support). It can be, should be completely re-written by cut-n-paste of the code in https://github.com/opencog/atomspace-rocks and then replacing RocksDB everywhere with PostgreSQL. (or any of your other favorite DB's ... as desired). The result will be a smaller, faster and fully compliant postgres backend.

This is not hard. Tedious, perhaps -- there's a LOT to do, a zillion places where the cut-n-pasted code needs to be touched. But there are no deep, conceptual stumbling blocks. The road ahead has been paved.