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
Do we need to warn API users that this also happens? If I want to quickly change hashers and have files hashed with SHA-1 that are big, will the change be slowed down by the verification step?