strip SessionContainer\ prefix from $name in Zend\Session\ContainerAbstr...

Docblocks indicate usage of SessionConainer\ prefix

    33  * Services use the prefix "SessionContainer\\":                                
    34  *                                                                              
    35  * <code>                                                                       
    36  * $container = $services->get('SessionContainer\captcha');                     
    37  * </code>                

and that normalizeContainerName strips off the prefix

    141      *                                                                          
    142      * Strips off the "SessionContainer\" prefix, and lowercases the name.      
    143      *    

This changeset implements the documented usage without breaking BC.