watchtower: convert JusticeKit to interface

In this commit, we convert the `JusticeKit` struct to an interface.
Then, we add two implementations of that interface:
1) The `legacyJusticeKit` which implements all the methods of
   `JusticeKit`
2) The `anchorJusticKit` which wraps the `legacyJusticeKit` and just
   re-implements the `ToRemoteOutputSpendInfo` method since.

watchtower/wtserver/server_test.go

@@ -30,7 +30,10 @@ var (
 
 	testnetChainHash = *chaincfg.TestNet3Params.GenesisHash
 
-	testBlob = make([]byte, blob.Size(blob.TypeAltruistCommit))
+	testBlob = make(
+		[]byte, blob.NonceSize+blob.V0PlaintextSize+
+			blob.CiphertextExpansion,
+	)
 )
 
 // randPubKey generates a new secp keypair, and returns the public key.