R1:1f3fcce6fe2d

Provide a cached class map query for making key-based class lookups more efficient

Summary:
Ref T11954. Depends on D16993. We have a couple of "look up the class for this key" queries which are costly enough to show up on a profile.

These aren't huge wins, but they're pretty easy. We currently do this like this:

```
$class_map = load_every_subclass();
return idx($class_map, $key);
```

However, we don't need to load EVERY subclass if we're only looking for, say, the Conduit method subclass which…