Perf: save ~9% of object allocations on heavy requests.

The per-thread registry is keyed on the class name, and each request for
the class name returns a new string. This is in the hot path for a lot
of Active Record behavior, so we easily accumulate thousands of repeated
strings.

To fix, we simply cache the key when the class is first extended with
the module.

TODO: Eliminate this module. The per-thread instance concept is common,
but this technique confuses and obfuscates.
Add a bunch of Relation -> Array delegate methods to the whitelist. This won't last - aim to switch back to a blacklist for mutator methods.