(stable) Manually set "max_allowed_packet" to 1GB for "mysqldump"

Summary:
We have one production instance with failing database backups since they recently uploaded a 52MB hunk. The production configuration specifies a 64MB "max_allowed_packet" in [mysqld], but this doesn't apply to mysqldump (we'd need to specify it in a separate [mysqldump] section) and mysqldump runs with an effective limit of the default (16MB).

We could change our production config to specify a value in [mysqldump], but just change it unconditionally at execution time since there's no reason for any user to ever want this command to fail because they have too much data.

Test Plan: Dumped locally, will verify production backup goes through cleanly.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18834