Mostly defuse DNS rebinding attack for outbound requests

Summary: Ref T6755. I'll add some notes there about specifics.

Test Plan:

• Made connections to HTTP and HTTPS URIs.
• Added some debugging code to verify that HTTP URIs were pre-resolved.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6755

Differential Revision: https://secure.phabricator.com/D12169