Change the default value of maxMessagePerRead to 16 in ChannelMetadata (

#13680)

Motivation:

We used 1 as the default value for maxMessagesPerRead in ChannelMetadata
which is a very bad default for performance reasons. Because of this we
usually explicit use 16 when constructing it. To eliminate problems when
we forgot to do so let's use 16 as default.

Modifications:

Change default to 16

Result:

Better performance by default

transport/src/main/java/io/netty/channel/ChannelMetadata.java

@@ -35,7 +35,7 @@ public final class ChannelMetadata {
      *                          again, such as UDP/IP.
      */
     public ChannelMetadata(boolean hasDisconnect) {
-        this(hasDisconnect, 1);
+        this(hasDisconnect, 16);
     }
 
     /**