Parallelize forward DNS name resolution

[dmiller-nmap]

This is a known source of slowness that should have been made a tracking issue long ago.

Nmap has a fast parallel DNS resolver, but it currently only works for PTR requests. For forward lookups (such as a list of domain names provided via -iL), it uses the system's own blocking resolver. We could really speed up scanning if we did these in batches instead, but it will require some re-engineering of how Nmap processes target inputs:

• Currently, name resolution is done in the various subclasses of NetBlock in TargetGroup.cc. This is to handle cases like example.com/24 (scan the /24 CIDR block that contains the address for "example.com").
• We'll likely need to implement a different intermediate class that takes target specifications and returns a stream/list/queue of net blocks.
• We need to extend the parallel DNS resolver in nmap_dns.cc to handle different request types. This functionality could be useful in other areas of Nmap, too, but don't worry about that yet.