Cleanup and clarification re cache_dir and state_dir (rename in tor_dirmgr)

While working on HSS intro req replay log perstence, I needed to make some decisions and did some digging to see how existing code did it. That found me some yaks.

There is a breaking change to tor-dirmgr here: the name of the cache_dir config parameter is changed to align with the name used in arti-*.