We were already using OrderedSets in the manager object, but strvs in the
configuration parsing code. Using sets gives us better scaling when many
domains are used.
In oss-fuzz #13059 the attached reproducer takes approximately 30.5 s to be
parsed. Converting to sets makes this go down to 10s. This is not _vastly_
faster, but using sets seems like a nicer approach anyway. In particular, we
avoid the quadratic de-unification operation after each addition.
