Send only one request at a time from ShapedRequest

Summary:
'this._request' was never set so 'waiting' was always false.
Result was that several requests were sent at once which wastes resources and
leads to weird bugs when responses don't arrive in sending order.

Blame Rev: D258

Test Plan:
Write a comment extremely fast, watch for requests sent.
Add sleep(5) for some inputs to DifferentialCommentPreviewController, verify
correct order.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1612