Big image rnd

Start more rendering engine when viewing a big image
see https://trac.openmicroscopy.org.uk/ome/ticket/7918

The value can be configured by modifying, in ```config/container.xml`` the following entry.
```
    <!-- The number of rendering engines to start when viewing a 
         big image. -->
    <entry name="/services/RE/worker" type="integer">4</entry>
```

To test: 
 * open a big image and check the retrieval speed of the tiles.