Rings are keeping a lock only for the list, which apparently doesn't
need anything more than an mt_list, so let's first turn it into that
before dropping the lock. There should be no visible effect.
A ring is used for the DNS code but slightly differently from the generic
one, which prevents some important changes from being made to the generic
code without breaking DNS. As the use cases differ, it's better to just
split them apart for now and have the DNS code use its own ring that we
rename dns_ring and let the generic code continue to live on its own.
The unused parts such as CLI registration were dropped, resizing and
allocation from a mapped area were dropped. dns_ring_detach_appctx() was
kept despite not being used, so as to stay consistent with the comments
that say it must be called, despite the DNS code explicitly mentioning
that it skips it for now (i.e. this may change in the future).
Hopefully after the generic rings are converted the DNS code can migrate
back to them, though this is really not necessary.
