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 tested by http://ci.openmicroscopy.org/view/5.1/job/OMERO-5.1-merge-integration-java/lastCompletedBuild/testngreports/integration/integration.ManagedRepositoryTest/.