Add database configuration source to the source stack

Summary:
Read configuration from the new database source.

This adds an extra MySQL connect + query to every page. They're very cheap so I think we can suffer them for now, but I'd like to put cache in front of this at some point. The difficulties are:

  - If we use APC, multi-frontend installs (Facebook) can't dirty it (major problem), and the CLI can't dirty it (fine for now, maybe a major problem later).
  - If we use Memcache, we need to add config stuff.
  - We could use APC in all non-Facebook installs if we can make it dirtyable from the CLI, but I don't see a reasonable way to do that.
  - We don't have any other caches which are faster than the database.

So I'll probably implement Memcache support at some point, although this is a lame excuse for it.

Test Plan: Added some config values via web UI, saw them active on the install.

Reviewers: btrahan, codeblock, vrana

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2221

Differential Revision: https://secure.phabricator.com/D4296