Load libraries before adding database config

Summary:
If your configuration overrides the connection adapter, we need to load libraries before we can setup the database config source.

Also lock this since it won't work when edited from the web anymore, and so sneaky users can't upload stuff and then edit their config to run arbitrary code.

Test Plan: See chatlog in #phabricator. This is a problem for Facebook only.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4498