Print request size when debugging Conduit calls

Summary:
See https://github.com/facebook/phabricator/issues/233

This is hopefully of general use when debugging the various crazy request size bugs we hit.

Test Plan: Ran arc diff --trace, saw request sizes.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3980