dns: rename domain to hostname

A follow-up commit will save the domain name on the request object but
we can't call that property 'domain' because that gets intercepted by
src/node.cc and lib/domain.js to implement the node.js feature of the
same name.

To avoid confusion, rename all variables called 'domain' to 'hostname'.
dns: set hostname property on error object

Make debugging and logging easier: when a DNS lookup for a hostname
fails, set the hostname as a property on the error object.

Fixes #5393.