Move connection cache from "AphrontDatabaseConnection"-level to "LiskDAO"-level

Summary:

• Currently, connections are responsible for connection caching. However, I want unit tests to be able to say "throw away the entire connection cache" with storage fixtures, and this is difficult/impossible when connections are responsible for the cache.
• The only behavioral change is that previously we would use the same connection for read-mode and write-mode queries. We'll now establish two connections. No installs actually differentiate between the modes so it isn't particularly relevant what we do here. In the long term, we should probably check the "w" cache before building a new "r" connection, so transactional code which involves reads and writes works (we don't have any such code right now).

Test Plan: Loaded pages, verified only one connection was established per database. Ran unit tests.

Reviewers: btrahan, vrana, jungejason, edward

Reviewed By: vrana

CC: aran

Maniphest Tasks: T140

Differential Revision: https://secure.phabricator.com/D2342