Optimize the database queries

[leofeyer]

This PR relates to #7450.

[aschempp]

Would it be useful to move $time = time() - (time() % 60); to something like \Date::minute()?

[leofeyer]

As discussed on Mumble on March, 26th, we should add a method Date::floorToMinute():

public static function floorToMinute($intTime=null)
{
    if ($intTime === null)
    {
        $intTime = time();
    }

    return $intTime - ($intTime % 60);
}

[leofeyer]

As discussed on Mumble on March, 26th, the indexes on the source and parent columns can be removed and a stand-alone index on the published column should be added.

[leofeyer]

Merged in 573b4b6.