Logback conversion (rebased onto develop)



This is the same as gh-889 but rebased onto develop.

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This PR moves any explicit use of log4j to logback. No `log4j.properties` or similar should remain in the repository, and it should in fact be possible to remove the log4j jars.

A next step will be to remove all hard-coded dependencies on any slf4j binding, perhaps by providing our own which delegates.

Testing instructions will be forthcoming. Certainly all of the merge/test jobs (openbytes, performance, full-repo, test_images_good) need to continue to work as expected.