feat: new BACKUP2 transfer protocol

New protocol streams .tar into iroh-net
stream without traversing all the files first.
Reception over old backup protocol
is still supported to allow
transferring backups from old devices
to new ones, but not vice versa.

src/peer_channels.rs

@@ -221,7 +221,7 @@ impl ChannelState {
 }
 
 impl Context {
-    /// Create magic endpoint and gossip.
+    /// Create iroh endpoint and gossip.
     async fn init_peer_channels(&self) -> Result<Iroh> {
         let secret_key = SecretKey::generate();
         let public_key = secret_key.public();