This massively simplifies transit locking behavior by pushing some
locking down to the Policy level, and embedding either a local or global
lock in the Policy depending on whether caching is enabled or not.
* Start work on passing context to backends
* More work on passing context
* Unindent logical system
* Unindent token store
* Unindent passthrough
* Unindent cubbyhole
* Fix tests
* use requestContext in rollback and expiration managers
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
