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
Merge pull request #1884 from joshmoore/11675-config-dispose

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