R1:0bf8e33bb6c1

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…