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 contains a reader concurrency semaphore for the tests, that they
can use to obtain a valid permit for reads. Soon we are going to start
working towards a point where all APIs taking a permit will require a
valid one. Before we start this work we must ensure test code is able to
obtain a valid permit.
