Message: make serializer field immutable

This requires two new pass-through constructors, but in return we
get rid of the setter.

core/src/main/java/org/bitcoinj/core/ChildMessage.java

@@ -33,6 +33,10 @@ public abstract class ChildMessage extends Message {
         super(params);
     }
 
+    public ChildMessage(NetworkParameters params, MessageSerializer serializer) {
+        super(params, serializer);
+    }
+
     public ChildMessage(NetworkParameters params, ByteBuffer payload) throws ProtocolException {
         super(params, payload);
     }

core/src/main/java/org/bitcoinj/core/Message.java

@@ -50,7 +50,7 @@ public abstract class Message {
     // The raw message payload bytes themselves.
     protected ByteBuffer payload;
 
-    protected MessageSerializer serializer;
+    protected final MessageSerializer serializer;
 
     @Nullable
     protected final NetworkParameters params;
@@ -65,6 +65,11 @@ public abstract class Message {
         this.serializer = params.getDefaultSerializer();
     }
 
+    protected Message(NetworkParameters params, MessageSerializer serializer) {
+        this.params = params;
+        this.serializer = serializer;
+    }
+
     /**
      * 
      * @param params NetworkParameters object.
@@ -99,17 +104,6 @@ public abstract class Message {
     protected void unCache() {
     }
 
-    /**
-     * Overrides the message serializer.
-     * @param serializer the new serializer
-     */
-    public void setSerializer(MessageSerializer serializer) {
-        if (!this.serializer.equals(serializer)) {
-            this.serializer = serializer;
-            unCache();
-        }
-    }
-
     /**
      * <p>Serialize this message to a byte array that conforms to the bitcoin wire protocol.</p>
      *

core/src/main/java/org/bitcoinj/core/PeerAddress.java

@@ -96,10 +96,9 @@ public class PeerAddress extends ChildMessage {
      * Construct a peer address from a memorized or hardcoded address.
      */
     public PeerAddress(NetworkParameters params, InetAddress addr, int port, BigInteger services, MessageSerializer serializer) {
-        super(params);
+        super(params, serializer);
         this.addr = Objects.requireNonNull(addr);
         this.port = port;
-        setSerializer(serializer);
         this.services = services;
         this.time = Optional.of(TimeUtils.currentTime().truncatedTo(ChronoUnit.SECONDS));
     }