checking connection inside loop

No, I'm talking about `connected` counter and mutex. Why create separate functions mysqlConnect and mysqlConnClose when you can add this logic here: 
https://github.com/percona/percona-agent/blob/release/mysql/mysql.go#L63
https://github.com/percona/percona-agent/blob/release/mysql/mysql.go#L96

This way everything that uses `percona-agent/mysql` package can benefit of this.