dak/show_new.py: use apply_async instead of map_async

dak/show_new.py: use apply_async instead of map_async

The map variant established multiple database connections in each worker
exceeding the maximum number of connections configured in PostgreSQL.
apply_async doesn't have this problem with our wrapper in DakProcessPool.

However as a regression we longer have a timeout and always have to wait for
the job to finish. This could be worked around by using the timeout function
for individual results.