Make storage.mysql-engine.max-size independent on max_allowed_packet

Summary:
I like systems that just work. It is possible to store files larger than max_allowed_packet in MySQL and we shouldn't demand it.

It also fixes a problem when file was smaller than storage.mysql-engine.max-size but its escaped version was larger than max_allowed_packet.

Test Plan: Reduced the size to 5e4, uploaded 90 kB file, checked the queries in DarkConsole, downloaded the file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3392