[keyserver] Add MySQL config to mirror production server

Summary:
Pulling these configs from the production MySQL server (see context here).

Depends on D4175

Test Plan:
Without these configs, I was not able to restore a backup from the production server. I was getting this error:

Got a packet bigger than 'max_allowed_packet' bytes

After this diff, the error went ahead and the backup was successfully restored.

Reviewers: palys-swm, atul

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4176