MirrorWebsite: make job scheduling less fragile
MirrorWebsite: make job scheduling less fragile

Up to now, we had an elaborate, clever, and hence pretty fragile way
to schedule mirror jobs. As a consequence, things broke down when we
tried it on file:// URLs.

Instead of pretending that we're clever, let's use a simple (and
therefore robust) way to schedule jobs.

And sure enough, the executor service gets shut down properly now.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

scijava/scijava-common