Pause headless rendering

I have my render dump frequency set so that it occurs about every 3 hours which seems reasonable to me. But sometimes I just want to halt the Chunky rendering process and dedicate all my computer's horsepower to something else temporarily, and then resume rendering afterwards. Unfortunately this can sometimes mean "wasting" several hours of rendering progress. Moreover, suspending and resuming the java process for Chunky on Windows "breaks" it and doesn't resume rendering after I un-suspend it. It would be very useful to have a way to pause rendering from the command line itself while doing headless, or perhaps a way to immediately save a render dump on-command.

Start by locating the headless rendering command-line entry point and the render dump handling. Determine how a pause/resume command or immediate render dump could preserve progress without suspending the Java process; done should include a documented, working way to control headless rendering safely.

Written by the indexing model from the issue text.