R1:8dccf05c4c1a

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…