ax(node): rename d to deduplicator in NewMessageDeduplicator (AX-1)

Single-letter variable `d` requires a reader to infer its type from
context. Rename to `deduplicator` — predictable names over short names
(RFC-CORE-008 §1).

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/transport.go

@@ -64,11 +64,11 @@ type MessageDeduplicator struct {
 // dedup.Mark(msg.ID)
 // if dedup.IsDuplicate(msg.ID) { continue }
 func NewMessageDeduplicator(ttl time.Duration) *MessageDeduplicator {
-	d := &MessageDeduplicator{
+	deduplicator := &MessageDeduplicator{
 		seen: make(map[string]time.Time),
 		ttl:  ttl,
 	}
-	return d
+	return deduplicator
 }
 
 // if dedup.IsDuplicate(msg.ID) { continue } // drop already-processed message