Auto reconnect to database if connection is lost for mysqli and pdo

[dirtyren]

Hello,

I've added auto reconnect support for the mysqli and PDO drivers. I was having a lot of problems having mysql has gone away errors and this solves this problem.
I think it be simple to implement for every other database.

[Mike-Benoit]

Sending a "SELECT 1" query to check if the connection is up for every other query that is executed is a non-starter unfortunately, as that would be horrible for performance.

Instead it needs to detect the disconnection by an error/exception and only retry at that point. PR #111 has something similar to that functionality if you want to take a look.