R1:d784bf1ea813

Make disk-based setup caches more correct (but slower)

Summary:
Fixes T9599. When APC/APCu are not available, we fall back to a disk-based cache.

We try to share this cache across webserver processes like APC/APCu would be shared in order to improve performance, but are just kind of guessing how to coordinate it. From T9599, it sounds like we don't always get this right in every configuration.

Since this is complicated and error prone, just stop trying to do this. This cache has bad performance anyway…