VRT_priv_task() inserts a priv_task if it does not exist.

In some cases, it is more efficient to only insert a priv_task under
certain conditions. These require an API function to only return a
priv_task if it exists. This is VRT_priv_task_lookup().

A TODO comment has been added for a sensible performance optimization,
avoiding one VRBT traversal for the VRT_priv_task() case. I would
address this after this suggestion got accepted, if so.