LP #1234346: include timestamp in slow query log file when initializing a new file

**Reported in Launchpad by Michael Coburn last update 04-10-2013 12:14:08

I think it would be convenient if we logged a timestamp when the slow query log file is initialized. Right now the file looks like this:

/usr/sbin/mysqld, Version: 5.5.24-55-log (Percona Server (GPL), Release rel26.0, Revision 256). started with:
Tcp port: 3306 Unix socket: /var/lib/mysql/mysql.sock
Time Id Command Argument

A simple date time string would be sufficient. Then at a glance I can tell when the file was initialized.