We want to switch from using a single limit to a dual soft/hard limit.
As a first step we switch the limit field of `query_class_config` to use
the recently introduced type for this. As this field has a single user
at the moment -- reverse queries (and not a lot of propagation) -- we
update it in this same patch to use the soft/hard limit: warn on
reaching the soft limit and abort on the hard limit (the previous
behaviour).
This struct will serve as a container of all the query-class
dependent configuration such as the semaphore to be used and the memory
limit for unlimited queries. As there is no good place to put this, we
create a separate header for it.
