ServiceCipher shouldn't implement/derive Clone

[rubdos]

ServiceCipher: Clone implies that I can concurrently process two messages that should've been processed sequentially. Even considering that the underlying storage mechanism is synchronised/mutex'd, that's not enough: the synchronisation mechanism cannot protect against readA-readB-writeA-writeB internally.

The simplest mitigation would be to Arc<Mutex> the complete insides of ServiceCipher, the coolest would be by considering the session that needs to be accessed:
we could lock only a single session to enforce serialized access.
That's a lot of code though.