Issue setup guidance recommending MySQLi and MySQL Native Driver

Summary:
Fixes T12994. We need MYSQLI_ASYNC to implement client-side query timeouts, and we need MySQLi + MySQL Native Driver to get MYSQLI_ASYNC.

Recommend users install MySQLi and MySQL Native Driver if they don't have them. These are generally the defaults and best practice anyway, but Ubuntu makes it easy to use the older stuff.

All the cases we're currently aware of stem from apt-get install php5-mysql (which explicitly selects the non-native driver) so issue particular guidance about php5-mysqlnd.

Test Plan:

• Faked both issues locally, reviewed the text.
• Will deploy to secure, which currently has the non-native driver.

Maniphest Tasks: T12994

Differential Revision: https://secure.phabricator.com/D19216