This ensures that we can safely rotate and modify configuration
parameters with multiple requests in flight.
As a side effect we also get a cache, which should provide a nice
speedup since we don't need to decrypt/deserialize constantly, which
would happen even with the physical LRU.
* Remove raw endpoint from transit
* Add multi-key structure
* Add enable, disable, rewrap, and rotate functionality
* Upgrade functionality, and record creation time of keys in metadata. Add flag in config function to control the minimum decryption version, and enforce that in the decrypt function
* Unit tests for everything
