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

This closes #204

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.

SHA-256 is still used for password-based key derivation (will be replaced with Argon2) and Fortuna.

Implementation is based on the BLAKE2b implementation from BouncyCastle, and is
therefore licensed under the BouncyCastle license (which will make future
upstreaming of the code easier).