Provide hooks to plugin custom connectors and avoid modification in core files

Current system demands changes in core files to extend core to make use of custom connectors. To overcome this issue, providing hooks to define custom classes in config_override.php.

Extend this support for sessions and cache connectors for now. We can extend in similar way for other areas.

Ex: Sessions are currently handled by PHP default session handler. This can be customized using runtime connector hook and avoid core file modifications. By defining them in config_override.php file array('session' => 'Vtiger_CustomSession_Handler')