Fix intermittent omero.LockTimeout-s in OmeroJava tests (see #11455)

This PR utilises the ``SimpleBackOff`` class to measure the average delay of the system on which OMERO.server is running. This is then used in defining the command callback timeout in the integration tests. This should hopefully fix the omero.LockTimeout which was thrown from ``AbstractServerTest.java`` from time to time and broke http://hudson.openmicroscopy.org.uk/view/All/job/OmeroJava-integration-stable/.

To test - verify that http://hudson.openmicroscopy.org.uk/view/All/job/OmeroJava-integration-stable/ is green. Always.