cachetop: various issues

We've had problems with curses before (and still do, #1045) but I've just spent far too long trying to capture a screenshot of cachetop output and failing. It doesn't pause on Ctrl-S, and there's no -C to give rolling output like with other top tools.

I'm filing this ticket to suggest:
1. adding to CONTRIBUTING_SCRIPTS.md: do not use curses.
2. ditching curses from this tool.

We could fix Ctrl-S and -C with cachetop, but then what else?

Start with the cachetop entry point and CONTRIBUTING_SCRIPTS.md, then review how curses currently controls its output. Compare the proposed removal with the stated Ctrl-S and -C needs in other top tools. Done requires an agreed scope for changing cachetop and the contributor guidance, with the resulting behavior defined.

Written by the indexing model from the issue text.