PCT-637: Query Analytics does not handle MySQL Restart

Done

However, I'm afraid that FileIntervalIter goroutine might access mysqlConn.Connect/Close/Set at the same time as the goroutine with check if mysql was restarted. @daniel-nichter What you think? Should I protect mysql.Connection with mutex?
I haven't looked in pt-agent again, but I don't think this was the original check because if we look once at MySQL and uptime is 60s, then MySQL is restarted but with new config that prevents agent from connecting for 120s, and user fixes that problem and agent connects and sees uptime 120s which is > 60s.  From agent's point of view (since it never stopped running), MySQL uptime should be 60s + 120s = 180s, then we see real uptime 120s < expected 180s and we know MySQL restarted.