Resolver "nameserver" object support. (#897)

Resolver "nameserver" object support. (#897)

* Resolver "nameserver" object support.

This turns the list of nameserver strings in the resolver into a tuple
of nameserver objects, which abstract away making queries to a
nameserver of a given type.

The resolver's legacy nameserver list is "enriched" into a tuple of
nameserver objects whenever it is set.  Note that you cannot mutate
the object other than by setting,
e.g. res.nameservers.append("1.2.3.4") will not work.

Error message accumulation has been updated to refer to the
nameservers using a descriptive text form.

* doco fix

* more doco fixes

* do enrichment at Resolution time

* require a later mypy, fix type issues

* add nameserver doc