R1:f45a13cff406

Improve settings caches on fast paths like Conduit

Summary:
Ref T11954. This reduces how much work we need to do to load settings, particularly for Conduit (which currently can not benefit directly from the user cache, because it loads the user indirectly via a token).

Specifically:

- Cache builtin defaults in the runtime cache. This means Phabricator may need to be restarted if you change a global setting default, but this is exceptionally rare.
- Cache global defaults in the mutable cache. This…