conn pool consistency fix

[pjain1]

Fixes a consistency bug where changes made in one DuckDB connection are not immediately reflected in other DuckDB connections. Looks like the problem is that In the go-duckdb driver, the Open function creates a new DB handle (using duckdb_open) and new connection handle (using duckdb_connect) each time. So the use of multiple DB handles seems to be the problem here. Thanks @begelundmuller for the analysis.

Driver now implements DriverContext so sql.DB will call OpenConnector to obtain a Connector and then invoke
that Connector's Connect method to obtain each needed connection instead of invoking Driver's Open method which was creating new C.duckdb_database each time. Now we create database object once on call to OpenConnector and call C.duckdb_connect on same database to obtain new connections on call to Connect.

[marcboeker]

LGTM and thanks for the PR. I've never had this use case so far but I'm glad you have it, so connection pooling is now possible 🙂

[pjain1]

Thanks for the quick review and merge.