Ref T8424. Fixes T7114. This was envisioned as a per-request cache for reusing interpreters, but isn't a good fit for that in modern Phabricator.
In particular, it isn't loaded by the daemons, but they have equal need for per-request caching.
Since I finally need such a cache for Spaces, throw the old stuff away before I built a more modern cache.
Also resolves T7114 by dropping filtering on $_SERVER. I'm pretty sure this is the simplest fix, see D12977 for a bit more discussion.