Raise PhabricatorShapedRequest request timeout

Summary:
We hit this very short (1s) timeout when the browser chooses to resolve all the
diff requests before the preview request. In the long term we could start the
preview request only after all the diff requests resolve, but this solves the
issue for now and there's no reason for such a short timeout.

The historical reason to have this timeout at all is that intern was megaflaky
and that's no longer a problem.

Test Plan:
Faked it so it would use a 1ms timeout the first time and then a 20s timeout;
got reasonable behavior.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran
Differential Revision: 329