This change allows users to bypass the selected DNS resolver for
specific domains. The allows, for example, to make calls to a local TLS
server by rerouting a given domain to 127.0.0.1.
The approach I've taken for the design is to wrap the resolver in an
outer service. This leads to a fair amount of boilerplate code mainly to
be able to explain the typing to the compiler. The actual business logic
is very simple for the number of lines involved.
Closes#561
8e5af459e5