Build speed-up (rebased onto develop)



This is the same as gh-611 but rebased onto develop.

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Saved from gh-569, these commits should significantly
increase the build times. This is done partly by
delaying file zipping until the release-all stage and
partly by only signing jars when necessary.

To test the full build, use docs/hudson/OMERO.sh but
individual developer workflows should also still function.