fix #10766: add API method for changing original files' hasher

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/10766 by adding a `ManagedRepository.setChecksumAlgorithm` method documented in https://github.com/mtbc/openmicroscopy/commit/33f7c04d6e957e7fcc214dd461ab4ee1913da05a and tested by http://ci.openmicroscopy.org/view/5.1/job/OMERO-5.1-merge-integration-java/lastCompletedBuild/testngreports/integration/integration.ManagedRepositoryTest/.
--no-rebase
Will add to the docs.

Decided against changing actual code: I'm doubtful about equalling the Guava hasher `putFile` speed for large image files with manual file-reading without using `FileChannel.map` and there is not yet any `MappedByteBuffer.unmap()` method (leading to the horror of `ome.util.PixelData.dispose()`) so will leave code as it is.