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
I'd guess the I/O time would be worse than the checksum time. Will mention in docs but also see if I can find both checksums with one read.