Use message headers hash to identify messages

app/src/main/java/eu/faircode/email/EntityMessage.java

@@ -102,6 +102,7 @@ public class EntityMessage implements Serializable {
     public Long forwarding; // obsolete
     public Long uid; // compose/moved = null
     public String msgid;
+    public String hash; // headers hash
     public String references;
     public String deliveredto;
     public String inreplyto;