Whether or not a class needs to run as a service is an implementation decision.

* Only show them as unread when they arrive out of order
* Mark all messages as read when sending a message

BdfList and BdfDictionary are no longer thread-safe, they require external locking. Metadata (which is the class that will be passed across API boundaries) is still thread-safe.

Addresses comments for !48.

Use BLAKE2s to generate tags.
KDF arguments for key rotation.
Frame IV format.