protofsm: use prefixed logger for StateMachine

So that we dont have to remember to add the `FSM(%v)` prefix each time
we write a log line.

protofsm/log.go

@@ -1,7 +1,7 @@
 package protofsm
 
 import (
-	"github.com/btcsuite/btclog"
+	"github.com/btcsuite/btclog/v2"
 	"github.com/lightningnetwork/lnd/build"
 )