Store encryption metadata with individual volumes

Encryption metadata is currently associated with (encrypted) volume types, which forces
snapshots, clones, and backups to all share the same volume type as the original volume. This
requirement, which is really an implementation artifact, leads to numerous special cases and
needless complexity.
This proposed change copies the encryption metadata from an encrypted volume type when a
volume is created. The encryption metadata is then stored with snapshots, clones, backups, etc.,
but there is no need to retain the original volume's type.
A side benefit of this change is the ability to modify the default encryption metadata (cipher, key
size, etc.) for a volume type since it will not affect existing volumes of that type. Currently
volume types cannot be updated or deleted once an encrypted volume has been created.