assist #11675: prevent some leaking direct buffers

This PR attempts to help http://trac.openmicroscopy.org.uk/ome/ticket/11675 in promptly cleaning up native memory buffers without resorting to requesting that the system perform a full garbage collection. Various image-centric operations should be attempted -- view, import, project, etc. -- to look for any being broken. This is not a great solution, but there is no good solution.

In inspecting the code, make absolutely sure that a `PixelData` object (or its `data`) upon which `dispose()` has been called is not plausibly subsequently accessed by any thread. Also, check that this PR causes no integration test failures for http://hudson.openmicroscopy.org.uk/view/All/job/OmeroJava-integration-develop/ or http://hudson.openmicroscopy.org.uk/view/All/job/OmeroPy-integration-develop/.

Leaked buffers will eventually be caught by garbage collection and finalization anyway, so this PR's coverage isn't complete. A clear present omission is `Plane2D`-related code.

Cc: @dpwrussell

Only one method, `ome.util.PixelData.dispose()`, uses the class `sun.nio.ch.DirectBuffer`. Even though it is useful, referencing this class is non-standard and **the ticket should remain open** until solved in some better way and the method's implementation revised.