A change in copystructure has caused some panics due to the custom copy
function. I'm more nervous about production panics than I am about
keeping some bad code wiping out some existing warnings, so remove the
custom copy function and just allow direct setting of Warnings.
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
