We have a sync Clear API to clear a pool, but no async counterpart. Now that we'll have NpgsqlDataSource, we'd call that from DisposeAsync. We could also optionally add static ClearPoolAsync and ClearAllPoolsAsync on NpgsqlConnection (for non-DbDataSource usage), but it's probably not important.

Note that it's very unlikely that closing connectors actually blocks, so this probably isn't critical - but we should do it to be 100% correct.