Fix broken test of marshalling tree in accordance with:
https://trac.openmicroscopy.org.uk/ome/ticket/12474

The test now correctly uses a read-only group and thus also asserts that
the received permissions reflect that
Fix permissions for tree projections

Makes use of a cache to avoid repeated querying

Temporary fix for:
http://trac.openmicroscopy.org.uk/ome/ticket/12474
Fixed bug where project permissions being used in place of dataset
Restored normal permissions parsing in tree

Convert the permission type in the workaround instead