ThreadSanitizer reported that a data race occurs when we attach to a
database and load its tables concurrently. Remove the custom code that
waits for all tables to load, and replace it with a local thread pool
and its wait() method.
This changes observable behavior: before, we would load the tables
sequentially when loading the system database, or performing an ATTACH
DATABASE query. Now we always load tables of ordinary databases in
parallel.
