luigi/compressed_graph: Tune -Xmx per task
Instead of passing '-Xmx 3000G' to every task, which prevented running tasks in parallel or to use Luigi's ability to schedule other memory-hungry tasks at the same time while respecting memory limits
This was tested on both the trivial test dataset and on a dataset with only origin and snapshot objects
Resolves #4767 (closed)
Part of #4775