RemoteIdentityChange

A change of remote's identity from previousIdentityKey (pinned) to presentedIdentityKey, as reported by SecureMessageClientException.IdentityChanged. Accepting it (SecureMessageClient.acceptRemoteIdentityChange) installs exactly presentedIdentityKey, and only while previousIdentityKey is still pinned. Only the client creates changes, so an application can accept only a key that a peer actually presented.