R1:a88b69a4b67c

Add a "setup" cache

Summary:
See T2062. This cache allows us to essentially implement this sort of block:

if (this_code_has_not_run_since_the_last_server_restart()) {
...
}

This will let us do setup checks automatically (i.e., without a specialized setup mode) without imposing hundreds of milliseconds of `git submodule status` and similar checks on every page load, even if an install does not have APC.

Broadly, the major goals here are:

- Reduce user errors and support costs related to…