Use HTTPS keepalives in HTTPSFuture if possible

Summary: cURL can manage keepalives as long as you reuse the connection. Since this class is synchronous (despite being a "future"), this is pretty easy. Make appropriate adjustments so we can get HTTPS keepalives.

Test Plan: Ran various "arc" commands under --trace with keepalives, observed dramatic performance improvements for calls 2..N.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2204