Add omero.pixeldata.dispose for static config (See #11675)

Since the cleaning should only be necessary under certain conditions
(most likely a 32-bit JVM), the disposing of the ByteBuffers held by
PixelData instances can be en-/disabled with omero.pixeldata.dispose

See: http://trac.openmicroscopy.org.uk/ome/ticket/11675

Testing:
 * Up the logging for `ome.util.PixelData` in `etc/logback.xm`l to `DEBUG`
 * Import `a.fake` or similar.
 * Check the log for `2013-12-10 08:38:21,839 DEBUG [                      ome.util.PixelData] (l.Server-3) omero.pixeldata.dispose set to true`
 * If you have a 32-bit JVM, possibly test on very large images. (This is **very** hard to reproduce)