This base class must be extended by storage gateways to be used with a
{@link eZ\Publish\Core\FieldType\GatewayBasedStorage} based storage
implementation.
The {@link setConnection()} method is called by the GatewayBasedStorage to
set the connection from the current persistence context.
Allows injection of the data storage connection to be used from external
source. This can be a database connection resource or something else to
define the storage, depending on the gateway implementation.
