Add compression to JSON-serialized input readers

Summary:
This compresses the JSON-serialized input readers with zlib.

A follow-up to: http://phabricator.khanacademy.org/D6147

Per @tom's comments, I'm going to leave `json_util.py` as is to minimize changes, since removing spaces in the `,` and `:` separators doesn't give us much. Simple enough!

Test Plan: Run locally and on znd

Reviewers: tom, mattfaus

Reviewed By: mattfaus

CC: jace, eliana, tom

Differential Revision: http://phabricator.khanacademy.org/D6215