rez-caching

Rez's two independent caches — the memcached-backed resolve cache and the on-disk package payload cache — the five settings that turn each one on, what invalidates an entry and what silently does not, rez-memcache and rez-pkg-cache for inspecting and flushing them, and the decision tree for 'I changed the package and nothing took effect'. Use when a resolve is slow, when a package change appears to be ignored, or when you need to prove a stale cache is or is not the cause. Covers Rez 3.4.0.