Bug report

Describe the bug

See https://forum.image.sc/t/exporting-measurements-in-qupath-v0-6-0-rc4/112103

To Reproduce

Steps to reproduce the behavior:

1. Create a project with several large data files (e.g. OS-2.ndpi with lots of cells detected)
2. Relaunch QuPath (to ensure the memory is cleared)
3. Open the project and before opening any image go to Measure → Export measurements...
4. Click 'Populate'
5. Monitor memory with VisualVM - it seems all the data is hanging around longer than it should

Expected behavior

Measurement exporter shouldn't hold on to objects from more than one image at a time.

Screenshots

From VisualVM after trying the steps above

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.6.0-rc4

Additional context

It relates to the use of PathObjectLazyValues and attempts to cache measurements.

Probably related to https://forum.image.sc/t/qupath-extremely-slow-exporting-detection-measurements/71154/26