Move the task creation into cache_create_db()

Move the task creation into cache_create_db()

The dns_cache_flush() drops the old database and creates a new one, but
it forgets to create the task(s) that runs the node pruning and cleaning
the rbtdb when flushing it next time.  This causes the cleaning to skip
cleaning the parent nodes (with .down == NULL) leading to increased
memory usage over time until the database is unable to keep up and just
stays overmem all the time.

(cherry picked from commit 79040a669ca26b75b06c25d27418cee7ab658013)