Compress requests from the Conduit client to Phabricator

Summary: Ref T13507. Enable compression in the body of Conduit requests if we have client support for it; we expect the server should always support it.

Test Plan: Created this revision, ran random "arc" commands that use Conduit. Added debugging code, saw payload size drop.

Maniphest Tasks: T13507

Differential Revision: https://secure.phabricator.com/D21073