Implement Keys

On top of PNI, because this needs libsignal-service-rs#310 which has updated dependencies.

Implement Keys response, with some additional bells and whistles.

By extension, this requies us to implement master key and storage server key as well.

• Tests for deriving storage service key
• Add Keys to registerSecondDevice to fix #652 (closed) and #656 (closed)
• Add additional checks for handling sync messages
  • Signal Desktop: MessageReceiver::handleSyncMessage(envelope, syncMessage) for inspiration
  • Only secondary devices can send sync request messages
  • All devices can respond to sync requests too
• Add PIN lock "support" (assert that it doesn't exist for the time being)
  • Out of scope -- see #149
• Generate master/storage keys when registering as primary device
• Sync master/storage keys from primary device when linking as secondary device
  • ClientActor migration sounds somewhat proper..?
• Move MasterKey and StorageServiceKey to libsignal-service
• Make handling the keys a trait
• Respond to Keys sync request
• libsignal-service-rs#310 has been merged and updated here
• All messages have been tested -- see linked PR