Don't waste too much cache by a single changeset

Summary:
We have max_allowed_packet 1 GiB but our replication dies if the query is longer than unknown value (it dies with 293 MB long query).

Anyway, there's no reason why we should not save the cache if you have small max_allowed_packet.

Test Plan: Lowered $size to 100, deleted cache from DB, displayed changeset, verified issued queries in DarkConsole, verified DB.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3390