Start one background worker per database

[Y--]

Following #544, remove the duckdb.motherduck_postgres_database GUC parameter and start a background worker for each PG database (*)

For another database to synchronize with the BGW to work:

• extension must be created: CREATE EXTENSION pg_duckdb;
• duckdb should be able to write in public schema: GRANT ALL ON SCHEMA public to duckdb;