Set max_allowed_packet to infinity for phabricator.

Summary:
We were seeing some errors in the logs, like:
   Got a packet bigger than 'max_allowed_packet' bytes
from the phabricator task daemon.  epriestley said:

   Increase `max_allowed_packet` to 9999999999999999 in my.cnf.  I'm
   not really sure why the option exists or why it's set so low by
   default.  We've been running with a infinite limit forever, I
   believe.  The root issue is that that particular commit changed a
   file in a way that more than 16MB of diff text was generated --
   e.g., adding a 17MB file, or replacing an 8MB file or similar.

To make things a bit easier to follow, I've moved all the mysql
settings that are there because it's what phabricator needs, to a
single conf file.  Right now it's just the STRICT_ALL_TABLES and
max_allowed_packet.

Test Plan:
Fingers crossed.  Will update toby and then see if the error goes away
from /var/tmp/phd/log/daemons.log

Reviewers: chris, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D8912