Consider adding an optional connection pool utility

The way to run multiple statements at the same time in DuckDB is using multiple connections. So, it could be helpful to expose a utility that manages a set of connections, making this recommended pattern easy to use.

Some care is needed because DuckDB connections carry semantics: they are the scope of temporary objects, context such as the current database and schema, variables, and named prepared statements. The user of a connection pool needs to be aware of these semantics and should likely avoid using these features. With appropriate documentation, however, a connection pool could still be useful.