Migrate `max_allowed_packet` and GD checks to new setup stuff

Summary: These are nonblocking warnings and can move to post-install.

Test Plan: Broke my environment and observed the warnings.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran, asherkin

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4505