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?
So, I should copy this: https://github.com/percona/percona-agent/blob/release/test/mock/null_mysql.go
rename to something like restart_mysql.go and adapt it to my needs? Still confused.